feat: do not automatically generate gapic_library targets for protos that don't have a service

We currently generate gapic_library targets in the BUILD file for all protos, including protos that only contain messages (e.g. do not contain a service). We do not need to do that for protos that do not contain services.

We could either parse the proto itself for a service, or look at the yaml file for APIs being referenced to determine whether gapic_library targets need to be included.