Crucible scrubber needs to tell Nexus when it is done scrubbing

[leftwo]

The scrubber code is written and will scrub a volume with a read only parent.
Nexus has an internal API endpoint that can remove a read only parent from a volume.

We now need just the bit of code for crucible upstairs to contact Nexus when a scrub is
done and tell it to remove the read only parent from a volume.

[leftwo]

The API on the Nexus side is done: oxidecomputer/omicron#2157

Remaining work is to update propolis to call this endpoint when it has finished with
scrubbing a volume

[leftwo]

This change has made it into propolis