Movable HTTPClient and fixing WiFiClient copy (#8237)

- =default for default ctor, destructor, move ctor and the assignment move
- use `std::unique_ptr<WiFiClient>` instead of raw pointer to the client
- implement `virtual std::unique_ptr<WiFiClient> WiFiClient::clone()` to safely copy the WiFiClientSecure instance, without accidentally slicing it (i.e. using the pointer with incorrect type, calling base WiFiClient virtual methods)
- replace headers pointer array with `std::unique_ptr<T[]>` to simplify the move operations
- substitute userAgent with the default one when it is empty
(may be a subject to change though, b/c now there is a global static `String`)

Allow HTTPClient to be placed inside of movable classes (e.g. std::optional, requested in the linked issue) or to be returned from functions. Class logic stays as-is, only the underlying member types are changed.

Notice that WiFiClient connection object is now copied, and the internal ClientContext will be preserved even after the original WiFiClient object was destroyed.

replaces #8236
resolves #8231
and, possibly #5734

libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.h

@@ -26,11 +26,12 @@
 #ifndef ESP8266HTTPClient_H_
 #define ESP8266HTTPClient_H_
 
-#include <memory>
 #include <Arduino.h>
 #include <StreamString.h>
 #include <WiFiClient.h>
 
+#include <memory>
+
 #ifdef DEBUG_ESP_HTTP_CLIENT
 #ifdef DEBUG_ESP_PORT
 #define DEBUG_HTTPCLIENT(fmt, ...) DEBUG_ESP_PORT.printf_P( (PGM_P)PSTR(fmt), ## __VA_ARGS__ )
@@ -151,13 +152,12 @@ typedef std::unique_ptr<TransportTraits> TransportTraitsPtr;
 class HTTPClient
 {
 public:
-    HTTPClient();
-    ~HTTPClient();
+    HTTPClient() = default;
+    ~HTTPClient() = default;
+    HTTPClient(HTTPClient&&) = default;
+    HTTPClient& operator=(HTTPClient&&) = default;
 
-/*
- * Since both begin() functions take a reference to client as a parameter, you need to 
- * ensure the client object lives the entire time of the HTTPClient
- */
+    // Note that WiFiClient's underlying connection *will* be captured
     bool begin(WiFiClient &client, const String& url);
     bool begin(WiFiClient &client, const String& host, uint16_t port, const String& uri = "/", bool https = false);
 
@@ -235,7 +235,15 @@ protected:
     int handleHeaderResponse();
     int writeToStreamDataBlock(Stream * stream, int len);
 
-    WiFiClient* _client;
+    // The common pattern to use the class is to
+    // {
+    //     WiFiClient socket;
+    //     HTTPClient http;
+    //     http.begin(socket, "http://blahblah");
+    // }
+    // Make sure it's not possible to break things in an opposite direction
+
+    std::unique_ptr<WiFiClient> _client;
 
     /// request handling
     String _host;
@@ -247,12 +255,14 @@ protected:
     String _uri;
     String _protocol;
     String _headers;
-    String _userAgent;
     String _base64Authorization;
 
+    static const String defaultUserAgent;
+    String _userAgent = defaultUserAgent;
+
     /// Response handling
-    RequestArgument* _currentHeaders = nullptr;
-    size_t           _headerKeysCount = 0;
+    std::unique_ptr<RequestArgument[]> _currentHeaders;
+    size_t _headerKeysCount = 0;
 
     int _returnCode = 0;
     int _size = -1;
@@ -264,6 +274,4 @@ protected:
     std::unique_ptr<StreamString> _payload;
 };
 
-
-
 #endif /* ESP8266HTTPClient_H_ */