refactor(pubsub): use common option initializers (#8454)
refactor(pubsub): use common option initializers (#8454)

Use `g::c::internal::PopulateCommonOptions()` and
`g::c::internal::PopulateGrpcOptions()` to initialize the Pub/Sub
default options.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp