feat: add support for universe domain (phase 1)

[bshaffer]

Requirements:

1. For all credentials: Create the universe_domain read property. The default value for the property is googleapis.com.
2. For Service Account Credentials: Read the universe_domain from the credentials file and expose the value through the property created above. If the credentials file does not have the universe_domain value, the property should use the default value googleapis.com.

[bshaffer]

@vishwarajanand we won't cut a release until after the freeze, but I'd love to get an approval on this before the EOY

[vishwarajanand]

Do you want to handle the case when universeDomain is an empty string? Or is it guaranteed to be having a valid value (eg. when always downloaded from gcp console)

[bshaffer]

Good question. The spec does not provide any guidance around empty strings for universe domain (go/cloudsdk-tpc-authlibs), except when it is a 404 error from the MDS (which is handled in phase 2).

If we added special handling for empty strings, it would be to throw an exception. But as an empty string for a universe domain will throw an error in the client libraries anyway (because we throw an error for a user-supplied empty string, and also for a mismatch), I can't think of a real important reason why we need to throw an exception now, and would prefer to leave it out (unless we can think of a specific case where it would be important to do so).

[vishwarajanand]

Lgtm