[TASK] Blueprints/Service instances should have support for joining and leaving

[drewstone]

Overview

If we're running a blockchain, it might be worthwhile to let a Blueprint developer create a blueprint where they specifically allow for permissionless joining/leaving of instances. For example, if I have an instance where I've selected "Any validators" or "These 100 validators" and only 10 of them are running right now, it should be possible for any operator satisfying the join condition to join the active service.