feat: add service renaming

[hongalex]

This PR adds support for service name overriding (renaming).

This PR cannot be merged until the new client.proto changes land to support the renamed_services field and the actual code in generator.getServiceNameOverride is uncommented.

Pieces of the generator that are currently not changed by service renaming:

• Custom operations (in genrest.go)
• gencli
• snippets

Diff for Pub/Sub is viewable here: https://github.com/googleapis/google-cloud-go/compare/main...hongalex:google-cloud-go:new-gapic-diff?expand=1

Fixes #1581

[codyoss]

Could you extend one of the existing tests to show this is working as expected?

[quartzmo]

I noticed the following generated doc in the provided diff:

// NewTopicAdminClient creates a new publisher client based on gRPC.

Note: "new publisher client".
This seems acceptable to me for most cases, but I suppose it could result in some confusion depending on the original name.

[hongalex]

Note: "new publisher client". This seems acceptable to me for most cases, but I suppose it could result in some confusion depending on the original name.

Yes, that was a one off where the code calls serv.GetName that I didn't catch before. I took a look again and it seems like the other references are fine. Please check the diff branch again.

[quartzmo]

I still found one usage in the diff, if you want to look into it:

 NewTopicAdminRESTClient creates a new publisher rest client.

[hongalex]

@quartzmo that should be fixed now

@codyoss I added a test that generates a stub of the gRPC client with service renaming, which I think is sufficient.