Support Migrate Tools' "includes" in migrations

Opened a quick MR with the minimum to get this working. I opted to place the include key right before source, process and destination as I would guess that those are the keys that will be mostly "included", but I'm not at all tied to the order.

Ah, I didn't think about updating the example and test migrations to include the include key. Also will need an update path, I guess. Would love some confirmation, though, first that I'm on the right track with this before fixing the tests.