Add SSL Session capability to speed reconnections (#5160)

SSL Sessions enable most of the SSL handshake to be skipped when both
client and server agree to use them.  Add a BearSSLSession class and
an optional setting to the SSL client to enable this.

Note that SSL sessions are unrelated to HTTP sessions.  They are
ephemeral and only relate to the SSL parameters, not anything at
the HTTP protocol level.

libraries/ESP8266WiFi/src/BearSSLHelpers.h

@@ -119,4 +119,21 @@ class BearSSLX509List {
     br_x509_trust_anchor *_ta;
 };
 
+// Opaque object which wraps the BearSSL SSL session to make repeated connections
+// significantly faster.  Completely optional.
+namespace BearSSL {
+  class WiFiClientSecure;
+};
+
+class BearSSLSession {
+  friend class BearSSL::WiFiClientSecure;
+
+  public:
+    BearSSLSession() { memset(&_session, 0, sizeof(_session)); }
+  private:
+    br_ssl_session_parameters *getSession() { return &_session; }
+    // The actual BearSSL ession information
+    br_ssl_session_parameters _session;
+};
+
 #endif