update the default type of credentials for async rest constructor

[ohmayr]

The default type of credentials within rest_asyncio.py.j2 needs to be updated to google.auth.aio.credentials.Credentials once we leverage google.auth.aio within the template.

Consequently, the default type of the base rest class needs to be updated to either Object, Any or Union[google.auth.aio.credentials.Credentials, google.auth.credentials.Credentials].