feat(gapic-common): Honor universe domain in stubs

[dazuma]

Partial support for universe domains in GAPICs. This pull request includes changes to gapic-common.

• Adds optional universe_domain and endpoint_template arguments to stub class constructors. The former is the universe domain in which the call should take place (falling back to the GOOGLE_CLOUD_UNIVERSE_DOMAIN environment variable, then defaulting to googleapis.com). The latter is a template of the endpoint host in which the universe domain component has been replaced with $UNIVERSE_DOMAIN$ (expressed in the constant Gapic::UniverseDomainConcerns::ENDPOINT_SUBSTITUTION). If the latter is not present, the existing endpoint argument is used.
• Implements determining the endpoint given the universe domain and template.
• Provides accessors for the universe domain and endpoint that the client object can use to implement its own accessors.
• Checks credential universe domain for mismatch against configured universe domain.
• Requires googleauth 1.9 which implements universe domain support.