[Bug] protobuf-java should be an optional dependency for Pulsar Java client libraries

[lhotari]

          > IMO, making protobuf-java optional is beyond the scope of this PR. It’s something worth discussing separately at a later stage.

It was always meant to be optional. It's already optional in the unshaded module. It was included as required dependency by mistake and this is why it's getting included in the shaded jar.

It don't think it's beyond the scope to fix the root cause of the issue.

Originally posted by @merlimat in https://github.com/apache/pulsar/pull/23468#discussion_r1803518970