fix: GCECredentials lazily fetches from the metadata server to ensure a universe domain is known

[dazuma]

As part of the logging work, to tamp down on the possible glut of logs on startup in high-traffic environments, we stopped eagerly fetching tokens from the client library credentials object. This was intended not to make any functional difference, because credential objects generally fetch their own tokens just-in-time when requested.

However, #508 revealed one case we missed: GCECredentials do not have a universe_domain set until the token is fetched, since the universe domain comes from the metadata service. Thus, client libraries would fail to initialize with a universe domain mismatch error. This happens only:

• for GCECredentials, as all other credential types get their universe domain from the input (e.g. json key file), and
• if the gapic-common gem is older than 0.23.0, because that version temporarily disabled universe domain checking for compute engine for unrelated reasons

Because the intent is for a future version of gapic-common to re-enable universe domain checking, and because some customers have not or cannot update gapic-common, we're going to work around this by causing GCECredentials to fetch its token just-in-time if its universe_domain is queried.

Fixes #508

[viacheslav-rostovtsev]

LGTM