Setting-based migrations

[BraveCaperCat]

Hi! I'm making this request because one of the users of a mod I'm a collaborator on wants to add a setting to the mod to disable specific mod compatibilities. (and by mod compatibilities, I mean using other people's planets instead of our own) They provided code and everything, but they also disabled the migration(s) for our planets -> other people's planets. The problem is that the migration only works once and can't be conditional based on startup settings. The idea is a conditional migration done when startup settings change to migrate from one prototype to another - but on a per-game basis. Like when you ignore the game wanting you to sync startup settings when loading the game - that's when the new lua-based migration should fire, being able to change prototypes into each other based on the startup settings changed.