This PR does a few things:

• Migrates our current sled_provision_state to a new sled_policy enum, which has more information (per RFD 457). This PR implements the expunged state, not the graceful removal state.
• Adds a sled_state enum, which describes Nexus's view of the sled. This PR adds the active and decommissioned states.
• Adds internal code to move around between valid states.
• Makes the blueprint execution code aware of sleds eligible for discretionary services.
• Adds tests for all of this new stuff, as well as valid and invalid state transitions -- and also makes sure that if we do end up in an invalid state, things don't break down.

Not done here, but in future PRs (to try and keep this PR a manageable size):

• We'll add the endpoint to mark the sled as expunged (this is an irreversible operation and will need the appropriate warnings): Define public endpoint for marking sled as expunged #5134
• We'll add blueprint code to start removing sleds: nexus expungement: assign an old Nexus instance's sagas to another Nexus instance #5136
• We'll also remove the sled time_deleted because it has a lifecycle too complicated to be described that way -- instead, we'll add a time_decommissioned field: Replace sled's time_deleted with time_decommissioned #5131

Questions

• Should we make sled_list hide decommissioned sleds by default?
• Do we need any unique indexes on the sled table -- maybe something like "each non-decommissioned sled ID should have a unique serial number"?

TODO:

• State transition tests.
• Blueprint tests.
• Update all the other places that need the policy.
• Region allocation tests.
• Ensure that sled state isn't decommissioned while trying to change sled policy.
• DB migrations + tests to ensure migrations happen successfully.