Consider splitting `@Config`, `@ConfigDefault`, and `Task` to a separate artifact, such as `embulk-util-config-task`

Some other util librarires sometimes want to prepare a predefined `Task` interface, but they have to depend on the entire `embulk-util-config` to do that. The dependency involves Jackson collaterally -- which takes developers closer to the dependency hell.

If another util library can only depend on `@Config`, `@ConfigDefault`, and `Task`, it'd be helpful.