Don't return `true` with `WiFiClientSecureBearSSL::connected()` when really disconnected

[mcspr]

ref. https://github.com/esp8266/Arduino/pull/8329#issuecomment-933913998 not tested (yet)

Apply the same condition as with normal WiFiClient - we are not connected when it's not possible to both write and read.

Implement separate methods for actual connection status and the internal ssl engine status and update methods that were previously using available() for this purpose

Update examples to check available() when the intent is to only read the data and not interact with the client in any other way. Also, use connect() as a way to notify errors, no need to check things twice