core: add RestartRandomizedDelaySec= service option

core: add RestartRandomizedDelaySec= service option

We already support exponential backoff for automatic restarts via
RestartSec=/RestartSteps=/RestartMaxDelaySec=, but there is no way to
randomize the restart delay. When many instances of a service fail at
the same time (e.g. because a shared resource briefly went away) they
are all restarted in lockstep, creating a thundering herd problem.

So this commit adds a simple `RestartRandomizedDelaySec=` service
option which is similar to the timer `RandomizedDelaySec=` and
adds a randomized restart delay.