[nexus-db-model] separate out SledUpdate from Sled

[sunshowers]

Sled consists of several columns that aren't controlled by sled-agent, and we
end up in this weird place where we have Sled instances that don't reflect
reality. I'm working on adding a provision_state column which is controlled
by the operator, and again for which sled-agent doesn't know.

Clean this up by defining a new struct, SledUpdate, which only contains the
columns sled-agent knows about. The other columns get defaults when
into_insertable is called.