Since there aren't any packages using this yet, it has been put in as _internal surface in #612

Once this is being used, we should move to a public surface with this.

• google.auth.transport._aiohttp_requests -> google.auth.transport.aiohttp_requests
• google.auth._jwt_async -> google.auth.jwt_async
• google.auth._credentials_async -> google.auth.credentials_async
• google.oauth2._service_account_async -> google.oauth2.service_account_async
• google.oauth2._credentials_async -> google.oauth2.credentials_async
• google.oauth2._id_token_async -> google.oauth2.id_token_async

Also, google/auth/__init__.py should be updated with the following:

from google.auth._default_async import default_async
 
__all__ = ["default", "load_credentials_from_file", "default_async"]