Introduce SSLClient library based on WiFiClientSecure and rework WiFiClientSecure to use it

[sgalabov]

Summary

This PR reworks WiFiClientSecure into SSLClient so that it can be used with any Client-derived class.
Then it goes on to modify WiFiClientSecure to work with SSLClient in conjunction with a normal WiFiClient.
The PR was tested with ESP32 WiFi as well as with a GSM module supported by TinyGSM.

Impact

Any Client-derived class can get SSL applied to it, e.g. WiFiClient, but also TinyGsmClient and any other clients.
Protocol Clients can then be put on top of SSL, e.g. PubSubClient or ArduinoHttpClient.
This may also allow for DTLS to be supported as well in the same style in the future, e.g. CoAP over both WiFi and GSM, etc.

[CLAassistant]

All committers have signed the CLA.

[me-no-dev]

Amazing work! Please allow some time to review and test :)

[me-no-dev]

You will need to edit CMakeLists.txt to fix the build error: https://github.com/espressif/arduino-esp32/runs/4608114525?check_suite_focus=true#step:4:41

[sgalabov]

You will need to edit CMakeLists.txt to fix the build error: https://github.com/espressif/arduino-esp32/runs/4608114525?check_suite_focus=true#step:4:41

I think it's done now, sorry.

[sgalabov]

I will close and reopen a new PR with just the inclusion of SSLClient lib for now, I will see if I can deal with the rest of the errors later...

[sgalabov]

Opened #6055 instead.