fix: add javadoc support for pom-only shaded modules

[nodece]

Motivation

When publishing the bk to the Maven central repo, you will get the following error:

Error:  Deployment failed: 
Error:  pkg:maven/com.***.bookkeeper/bookkeeper-server-shaded@4.16.8.0: Javadocs must be provided but not found in entries
Error:  pkg:maven/com.***.bookkeeper/bookkeeper-server-tests-shaded@4.16.8.0: Javadocs must be provided but not found in entries
Error:  pkg:maven/com.***.distributedlog/distributedlog-core-shaded@4.16.8.0: Javadocs must be provided but not found in entries
Error:  pkg:maven/com.***.bookkeeper.metadata.drivers/jetcd-core-shaded@4.16.8.0: Javadocs must be provided but not found in entries

Changes

• add javadoc support for pom-only shaded modules
• control javadoc generation by maven.javadoc.skip

[nodece]

@lhotari @merlimat could you have a chance to review?