Add support for `default_host` in `OperationsRestAsyncTransport`

[parthea]

Support for default_host is not yet supported in OperationsRestAsyncTransport which was added in #700.

The reason that it was not included in #700 is because it is not yet supported in google-auth. default_host exists here in class AuthorizedSession but there is no default_host in the async version here in AsyncAuthorizedSession