fix: javadoc plugin is now configured to fail on error

[akafredperry]

- What I did
Addressed Javadoc errors and configured linters to fail the build going forward.

closes #339

- How I did it

• Removed the maven-javadoc-plugin config which allowed javadoc violations to be ignored. Missing javadoc is still permitted at this point because the maven plugin does not support "fine grained" configuration and it is not appropriate to javadoc-ing every single method and field at the moment
• Added Javadoc modules to the checkstyle config so that javadoc-ing public interfaces and classes is enforced by the checkstyle plugin
• Added javadoc comments for all classes and interfaces
• Moved DateUtil and FileUtil into at_utils module as these at not used in at_client
• Adjusted error response parsing in Secondary, the error codes should be encapsulated in the typed exception classes
• Removed AtNotYetImplementedException - jdk UnsupportedOperationException will suffice
• Removed RemotedLookupException - unused and not in at protocol specification
• Removed ResponseTransformer interface - jdk Function interface will suffice

- How to verify it
Checkout the branch and run the following command

mvn clean install

The maven build lifecycle will run all the tests and validate (this include spotless, checkstyle) and package will generate the javadoc

Note: this is covered by the CI test and build check

- Description for the changelog
fix javadocs and enforce class comments going forward