Provide request_class, response_class for httpx.Client

[q0w]

Summary

Closes #717

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[T-256]

Lets have it ✌️

[tomchristie]

Presumably also response_class, right?
Also I guess move this to the end of the parameters.

[q0w]

Presumably also response_class, right?

I thought about that, but baseclient has no logic related with responses, so response_class is now added only in Client for passing to transports. I can add response_class, if needed.

[trim21]

httpx.Reponse.raise_for_status() need to return Self instead of Response, which is added in py311.

So we need typing_extensions.Self in lower version.

or we can use def raise_for_status(self: Type[T]) -> T: instead.

Should also consider add a generic typing for response type and request type of client instance.

[q0w]

generic type for request and response type

#717 (comment)