Feature/httpx2 instrumentor (#4730)

* httpx2 instrumentation support

* small readme edit

* Add docstring for httpx2

* doc updates

* ensure that if only httpx2 is available, that the httpx instrumentor isn't backed by httpx2 classes

* feedback: httpx protocol to type checking block

* feedback: remove top level fail, fix annotations

* feedback: move class definitions to the same conditions, fix docgen and genned workflows

* feedback: pull out the client instrumentor

* tox -e generate

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Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>