Enable constructing GoogleServiceAccountClient with Credentials.from_service_account_info method #532

When initializing a GoogleServiceAccountClient, it expects Service Account JSON key file but there is no way to pass in the key file content as dictionary (e.g. after reading out from a secret store) because the constructor is limited to use the google.oauth2.service_account.Credentials.from_service_account_file()class method. Can this constructor be extended with .from_service_account_info() method, too?

https://google-auth.readthedocs.io/en/master/reference/google.oauth2.service_account.html#google.oauth2.service_account.Credentials.from_service_account_info