With this commit one can request compute reconfiguration
from the running `compute_ctl` with compute in `Running` state
by sending a new spec:
```shell
curl -d "{\"spec\": $(cat ./compute-spec-new.json)}" http://localhost:3080/configure
```
Internally, we start a separate configurator thread that is waiting on
`Condvar` for `ConfigurationPending` compute state in a loop. Then it does
reconfiguration, sets compute back to `Running` state and notifies other
waiters.
It will need some follow-ups, e.g. for retry logic for control-plane
requests, but should be useful for testing in the current state. This
shouldn't affect any existing environment, since computes are configured
in a different way there.
Resolvesneondatabase/cloud#4433
