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