Update type hints for credentials in the async client layer

[ohmayr]

Update the credentials type hint in the async client constructor to include asynchronous credentials i.e. async credentials should also support google.auth.aio.credentials.Credentials which are required by the async rest transport.

For now, the type hint isn't updated in the client layer and mypy warnings are silenced by adding # type: ignore in the async rest transport constructor. This can be looked up by searching for this issue.

Related issue: https://github.com/googleapis/gapic-generator-python/issues/2173