feat: Support setting the universe domain

[dazuma]

Spanner unfortunately needed a bit of a hack for proper universe_domain behavior. Normally, we just pass the universe domain down into the gapic-common layer and it will generate the correct endpoint host name. However, Spanner uses a prebuilt gRPC channel as a credential because of how it configures gRPC, which means we need to know the endpoint up front in order to construct that channel. Thus, we are currently copying from gapic-common (and settings from the GAPIC) to resolve the universe domain, and thus the endpoint. Later we will need to refactor things a bit to expose this logic either in the gapic-common layer or in the generated GAPIC, so the Spanner handwritten client can just call it directly.