opentelemetry-instrumentor-httpx: support strict typing #4818

This is a followup to #4730 (comment)

We should see if we can improve the typing story here. In service of that, we should add opentelemetry-instrumentation-httpx here...

https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/tox.ini#L1186

and do a pass over the instrumentor's typing as a whole.