Add BearSSL client and server, support true bidir, lower memory, modern SSL (#4273)

BearSSL (https://www.bearssl.org) is a TLS(SSL) library written by
Thomas Pornin that is optimized for lower-memory embedded systems
like the ESP8266. It supports a wide variety of modern ciphers and
is unique in that it doesn't perform any memory allocations during
operation (which is the unfortunate bane of the current axTLS).

BearSSL is also absolutely focused on security and by default performs
all its security checks on x.509 certificates during the connection
phase (but if you want to be insecure and dangerous, that's possible
too).

While it does support unidirectional SSL buffers, like axTLS,
as implemented the ESP8266 wrappers only support bidirectional
buffers. These bidirectional buffers avoid deadlocks in protocols
which don't have well separated receive and transmit periods.

This patch adds several classes which allow connecting to TLS servers
using this library in almost the same way as axTLS:
BearSSL::WiFiClientSecure - WiFiClient that supports TLS
BearSSL::WiFiServerSecure - WiFiServer supporting TLS and client certs

It also introduces objects for PEM/DER encoded keys and certificates:
BearSSLX509List - x.509 Certificate (list) for general use
BearSSLPrivateKey - RSA or EC private key
BearSSLPublicKey - RSA or EC public key (i.e. from a public website)

Finally, it adds a Certificate Authority store object which lets
BearSSL access a set of trusted CA certificates on SPIFFS to allow it
to verify the identity of any remote site on the Internet, without
requiring RAM except for the single matching certificate.
CertStoreSPIFFSBearSSL - Certificate store utility

Client certificates are supported for the BearSSL::WiFiClientSecure, and
what's more the BearSSL::WiFiServerSecure can also *require* remote clients
to have a trusted certificate signed by a specific CA (or yourself with
self-signing CAs).

Maximum Fragment Length Negotiation probing and usage are supported, but
be aware that most sites on the Internet don't support it yet.  When
available, you can reduce the memory footprint of the SSL client or server
dramatically (i.e. down to 2-8KB vs. the ~22KB required for a full 16K
receive fragment and 512b send fragment).  You can also manually set a
smaller fragment size and guarantee at your protocol level all data will
fit within it.

Examples are included to show the usage of these new features.

axTLS has been moved to its own namespace, "axtls".  A default "using"
clause allows existing apps to run using axTLS without any changes.

The BearSSL::WiFi{client,server}Secure implements the axTLS
client/server API which lets many end user applications take advantage
of BearSSL with few or no changes.

The BearSSL static library used presently is stored at
https://github.com/earlephilhower/bearssl-esp8266 and can be built
using the standard ESP8266 toolchain.

libraries/ESP8266WiFi/src/WiFiClientSecureAxTLS.h

@@ -0,0 +1,100 @@
+/*
+  WiFiClientSecure.h - Variant of WiFiClient with TLS support
+  Copyright (c) 2015 Ivan Grokhotkov. All rights reserved.
+  This file is part of the esp8266 core for Arduino environment.
+
+
+  This library is free software; you can redistribute it and/or
+  modify it under the terms of the GNU Lesser General Public
+  License as published by the Free Software Foundation; either
+  version 2.1 of the License, or (at your option) any later version.
+
+  This library is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public
+  License along with this library; if not, write to the Free Software
+  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+
+*/
+
+#ifndef wificlientsecure_h
+#define wificlientsecure_h
+#include "WiFiClient.h"
+#include "include/ssl.h"
+
+
+namespace axTLS {
+
+class SSLContext;
+
+class WiFiClientSecure : public WiFiClient {
+public:
+  WiFiClientSecure();
+  ~WiFiClientSecure() override;
+
+  int connect(IPAddress ip, uint16_t port) override;
+  int connect(const String host, uint16_t port) override;
+  int connect(const char* name, uint16_t port) override;
+
+  bool verify(const char* fingerprint, const char* domain_name);
+  bool verifyCertChain(const char* domain_name);
+
+  uint8_t connected() override;
+  size_t write(const uint8_t *buf, size_t size) override;
+  size_t write_P(PGM_P buf, size_t size) override;
+  size_t write(Stream& stream); // Note this is not virtual
+  int read(uint8_t *buf, size_t size) override;
+  int available() override;
+  int read() override;
+  int peek() override;
+  size_t peekBytes(uint8_t *buffer, size_t length) override;
+  void stop() override;
+
+  bool setCACert(const uint8_t* pk, size_t size);
+  bool setCertificate(const uint8_t* pk, size_t size);
+  bool setPrivateKey(const uint8_t* pk, size_t size);
+
+  bool setCACert_P(PGM_VOID_P pk, size_t size);
+  bool setCertificate_P(PGM_VOID_P pk, size_t size);
+  bool setPrivateKey_P(PGM_VOID_P pk, size_t size);
+
+  bool loadCACert(Stream& stream, size_t size);
+  bool loadCertificate(Stream& stream, size_t size);
+  bool loadPrivateKey(Stream& stream, size_t size);
+
+  void allowSelfSignedCerts();
+
+  template<typename TFile>
+  bool loadCertificate(TFile& file) {
+    return loadCertificate(file, file.size());
+  }
+
+  template<typename TFile>
+  bool loadPrivateKey(TFile& file) {
+    return loadPrivateKey(file, file.size());
+  }
+  
+  template<typename TFile>
+  bool loadCACert(TFile& file) {
+    return loadCACert(file, file.size());
+  }
+
+friend class WiFiServerSecure; // Needs access to custom constructor below
+protected:
+  // Only called by WiFiServerSecure
+  WiFiClientSecure(ClientContext* client, bool usePMEM, const uint8_t *rsakey, int rsakeyLen, const uint8_t *cert, int certLen);
+
+protected:
+    void _initSSLContext();
+    int _connectSSL(const char* hostName);
+    bool _verifyDN(const char* name);
+
+    std::shared_ptr<SSLContext> _ssl = nullptr;
+};
+
+};
+
+#endif //wificlientsecure_h