Report power state transitions to MGS

Basically, this commit glues together the changes from
#2064 and
oxidecomputer/management-gateway-service#390. The
`SpHandler::set_power_state` implementation for compute sleds in the
`control-plane-agent`
task now report the outcome of power state transitions using the new
`gateway_messages::PowerStateTransition` type added in oxidecomputer/management-gateway-service#390.

This fixes oxidecomputer/management-gateway-service#270, and closes
oxidecomputer/management-gateway-service#271.

We'll need to update MGS proper (in the Omicron repo) to pick up the new
`gateway-messages` changes, as well.

Author: hawkw