[sdk-platform-java] Implement downstream javadoc generation test on sdk-platform-java

To help catch javadoc errors upstream, implement a downstream check on the generator similar to this one: https://github.com/googleapis/java-shared-config/pull/662/ that checks javadoc generation on a couple of libraries to ensure javadoc generation works.

In terms of what the javadoc generation should test for:

1) The current cloud RAD generation uses Java11 with this doclet: https://github.com/googleapis/java-docfx-doclet/tree/main/third_party.

2) In general, to account for self-service clients, the plain `mvn javadoc:aggregate` command should also pass. This should be checked on Java 17, and possibly Java 11 as well. If we migrate the cloud RAD generation to use Java17, then I think we don't need to check for Java11 anymore, but that will be a separate project.