FirebaseArduino: switch to ArduinoHttpClient

[proppy]

Preliminary work to port the library to use ArduinoHttpClient.
This will enable the library to be portable across arduino cores implementing the Client Interface.

• ✔️ compile for esp8266
• ✔️ compile for esp32
• ✔️ CRUD tested on esp8266
• ✔️ CRUD tested on esp32
• ⚠️ streaming not tested on esp8266
• ⚠️ streaming not tested on esp32

Note this breaks the following libraries/sketches:

• FirebaseCloudMessaging
• contrib/modem
• contrib/things

And obsolete the following classes:

• Firebase
• FirebaseHttpClient

Testing welcome 🎠

Related #344 #220

/cc @ed7coyne @kotl

[kotl]

You may remember this, but I wanted to remind you that for ESP8266 implementation, please be extra careful about our use of workaround for ESP8266 of modifying _canReuse even after it was set to false in

https://github.com/esp8266/Arduino/blob/master/libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp#L1020

Through these lines:
if(headerName.equalsIgnoreCase("Connection")) {
_canReuse = headerValue.equalsIgnoreCase("keep-alive");
}
(Since RTDB tends to not send "keep-alive" value)

We change _canReuse value early enough, so that before HTTPClient::end is called, it is set to true.
However, we can't override HTTPClient::end() since it's not virtual and luckily for us, we never call HTTPClient::end, and it is also not called in HTTPClient implementation, therefore when we call ForceReuseHTTPClient::end, it works.

We will need to have same workaround for ESP8266 implementation using ArduinoHttpClient if RTDB is still acting in the same way.

[proppy]

@kotl Oh right, I forgot about this!

We're lucky as It seems that ArduinoHttpClient header handling doesn't parse "Connection: close":
https://github.com/arduino-libraries/ArduinoHttpClient/blob/master/src/HttpClient.cpp#L763

[proppy]

@kotl did you give it a try?