Java: Standardize hooks + middleware implementation with other languages - #1519

I noticed today that Java had been done something a little weird in that
instead of hooks/middleware like in Go, we ended up with a broad
`Extension` interface that does everything.

Here, give Java a more normal implementation compared to other
languages. We have specific hooks/middleware like:

    public interface HookInsertBegin extends Hook {
      /**
       * Runs in the insertion's transaction. Throwing fails and rolls back the insertion, including
       * application writes made using this connection. Options are the caller's overrides, before
       * merging with job-type defaults.
       */
      void insertBegin(Connection connection, JobType<?> type, Object args, InsertOptions options)
          throws Exception;
    }

Which extends from a base like `Hook`:

    public interface Hook extends Plugin {}

Which extends from `Plugin` so that everything can be set together:

    public interface Plugin {}