Add static code for supporting universe domain (to be used in both GAPICs and Apiary)

[ohmayr]

Universes other than the default "googleapis.com" need to be supported in both GAPICs and Apiary. The following code was added to GAPICs but can be moved to python-api-core:

def _get_universe_domain(
 client_universe_domain: Optional[str], universe_domain_env: Optional[str]
) -> str: # temp disable Optional
 """Return the universe domain used by the client.

 Args:
 client_universe_domain (Optional[str]): The universe domain configured via the client options.
 universe_domain_env (Optional[str]): The universe domain configured via the "GOOGLE_CLOUD_UNIVERSE_DOMAIN" environment variable.

 Returns:
 str: The universe domain to be used by the client.

 Raises:
 ValueError: If the universe domain is an empty string.
 """
 universe_domain = _DEFAULT_UNIVERSE
 if client_universe_domain is not None:
 universe_domain = client_universe_domain
 elif universe_domain_env is not None:
 universe_domain = universe_domain_env
 if len(universe_domain.strip()) == 0:
 raise _Empty_Universe_Error
 return universe_domain


def _compare_universes(
 client_universe: str, credentials: Union[ga_credentials, oauth2_credentials]
) -> bool:
 """Returns True iff the universe domains used by the client and credentials match.

 Args:
 client_universe (str): The universe domain configured via the client options.
 credentials Union[google.auth.credentials.Credentials, oauth2client.client.Credentials]: The credentials being used in the client.

 Returns:
 bool: True iff client_universe matches the universe in credentials.

 Raises:
 ValueError: when client_universe does not match the universe in credentials.
 """
 credentials_universe = getattr(credentials, "universe_domain", _DEFAULT_UNIVERSE)

 if client_universe != credentials_universe:
 raise _UniverseMismatchError(client_universe, credentials_universe)
 return True