Support hooking into the event that a worker finishes a build on the worker side

Motivation: For overprovisioning disk space on Debusine workers, they have to return space every now and then. This happens using fstrim. By default, fstrim runs once a week and that tends to be insufficient for the workload at hand. Whenever a worker finishes a build, it will release most of its disk space consumption and that would be a good moment to invoke fstrim.

Debusine workers should provide a mechanism for an administrator to hook into the event that a work request finishes.