Public all the migrators

[lars-erik]

I'm doing very custom things with a replacement for Vorto.
It is impossible without making the actual migration handlers replacable and preferably inheritable.
(Maybe Vorto shouldn't be so tightly coupled through ISyncReplacablePropertyMigrator?)

For now this is WIP and possibly way to hacky to mandate pulling, but wanted to make the PR for visibility and possible discussion.

Will update when my PoC works. 🙃

[lars-erik]

I'm un-WIP'ing this. Have not found anything else I need in order to migrate a completely bonkers structure. Let me know if I should rebase or merge. (Or give up. 🙈)