Closes #3111

MutableMapping and Awaitable are slightly more permissible types (I think, as they allow the previous Dict and Coroutine types) but more importantly they match up to Starlettes tpyes

Summary

Hello! When upgrading to the latest version of Httpx we found that our test client was producing types errors (but fine at run time), we use FastAPI which is Starlette under the hood:

AsyncClient(transport=ASGITransport(app=app), base_url="http://test")

Argument of type "FastAPI" cannot be assigned to parameter "app" of type "_ASGIApp" in function "__init__"
  Type "FastAPI" cannot be assigned to type "_ASGIApp"
    Parameter 3: type "_Send" cannot be assigned to type "Send"
      Type "_Send" cannot be assigned to type "Send"
        Parameter 1: type "Message" cannot be assigned to type "Dict[str, Any]"
          "Message" is incompatible with "Dict[str, Any]"Pylance[reportArgumentType](https://github.com/microsoft/pyright/blob/main/docs/configuration.md#reportArgumentType)

This might be a bug in pylance not understanding the nested mutable mappings and their compatibililty with dicts, but this change makes httpx more flexible with other ASGI apps, and matches for example Starlette's ASGI app types: https://github.com/encode/starlette/blob/7533b61a34f8b0dcb5fc35c717d458439f0baf18/starlette/types.py#L10

So the consistency might be worth it?

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. (covered by existing tests)
• I've updated the documentation accordingly.