(note: follow-up to #1193)

Currently BufferRecycler passed to JsonGenerator and JsonParser is explicitly accessed from RecyclerPool configured with JsonFactory. But there are cases where jackson-databind has already allocated an instance for use (currently mostly for output aggregation) and it would make sense to allow that to be used instead.
Let's do this by checking if input source/output target implements BufferRecycler.Gettable to expose pre-configured BufferRecycler.

EDIT: since this is only needed for output side (JsonGenerator), initial implementation is just for this.
If need arises can make changes to input side.