[sdk-platform-java] Enhance checkstyle plugin to check for missing Javadoc

**Is your feature request related to a problem? Please describe.**
We should check and enforce that there is a javadoc comment on public methods.

**Describe the solution you'd like**
The checkstyle plugin should throw errors if there are missing javadoc comments.

**Additional context**
https://github.com/googleapis/gax-java/pull/1880/files#r1034266172