Configurable actor and worker tGPS #1617

#1560 raised ateom's drain budget after SIGTERM from 1 minute to 30 minutes, which is the window an actor has to be suspended before ateom kills it. The worker pod's tGPS is hardcoded as 1 hour today.

Workloads differ. Some actors suspend within seconds, some may run for longer without a proper suspend point. A fixed 30 minutes maybe enough for 99% case but not for outliers, and users have no way to tune either number.

We may consider:

• ActorTemplate.terminationGracePeriodSeconds: how long this actor gets after SIGTERM before ateom kills it, with default 30 mins.
• WorkerPool.template.terminationGracePeriodSeconds: ceiling grace period for worker pods from this WorkerPool.