Java: Standardize hooks + middleware implementation with other languages

[brandur]

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 {}