Implement BaseRequest.get_extra_info()

[asvetlov]

It is a simple shortcut for request.transport.get_extra_info().

request.transport will go eventually.

Documentation update and a simple test are required.

[rbialon]

Hi @asvetlov ,
I'd like to work on this.
To clarify, while request.transport will go eventually, request._protocol.transport will stay and is still the right resource to invoke get_extra_info() on?

[asvetlov]

Correct.
It request._protocol.transport will go as well but the test suite will have a test for request.get_extra_info() -- we'll fix it immediately.

I don't like using request.transport.get_extra_info() because I want to raise deprecation warning for .transport property soon, much earlier before the actual removal.

[asvetlov]

Fixed by #4196