Generate proto-only libraries

[JoeWang1127]

The generate_library.sh should be able to generate libraries which don't have a java_gapic_library bazel target.

[blakeli0]

I would call them proto-only libraries, as they are still generated through the whole GAPIC toolchain. In addition, alloydb-connector does not have a gRPC folder as well, so we should not try to run gRPC plugin as well.
Also some other libraries like java-iam have proto and gRPC folder, but no gapic client folder. We should take those scenarios into consideration as well.

[JoeWang1127]

I would call them proto-only libraries, as they are still generated through the whole GAPIC toolchain.

Thanks for pointing it out, I've changed the title.

In addition, alloydb-connector does not have a gRPC folder as well, so we should not try to run gRPC plugin as well.

I noticed that even if I run the grpc plugin with the protos, there's no java file generated. I can simply delete the grpc-* folder if this folder is empty.

Also some other libraries like java-iam have proto and gRPC folder, but no gapic client folder. We should take those scenarios into consideration as well.

java-iam has another issue, I need to exclude google/iam/v1/logging.