Return a flag in disk GET to denote if deletion is allow

Currently clients have to replicate [this logic](https://github.com/oxidecomputer/omicron/blob/e5704d7f343fa0633751527dedf276409647ad4e/nexus/src/db/datastore.rs#L2103) to deduce whether a disk can be deleted. It'd easy downstream implementation if we just encoded this into the disk response. 