auth: configure_mtls_channel() discards existing session configuration

In `AsyncAuthorizedSession.configure_mtls_channel()` (`packages/google-auth/google/auth/aio/transport/sessions.py`), configuring mutual TLS replaces `self._auth_request` with a newly constructed `aiohttp.ClientSession` and `aiohttp.TCPConnector`:

```python
connector = aiohttp.TCPConnector(ssl=ssl_context)
new_session = aiohttp.ClientSession(connector=connector)
old_auth_request = self._auth_request
self._auth_request = AiohttpRequest(session=new_session)
```

Because `new_session` is instantiated from scratch with default arguments, we lose any custom configurations provided by callers on the initial session, such as connection pool limits, timeouts, etc.