Add support for shutdown-after

[FedericoPonzi]

Motivation and Context

Fixes #180

Description

shutdown-after = list<ServiceName>: Shut down this service only after the listed services have stopped.
This allows controlling the order in which services are terminated during a graceful shutdown.
For example, if service vpn has shutdown-after = ["api.toml"], then vpn will only receive its termination
signal after api has fully stopped. A second SIGTERM (forceful shutdown) bypasses this ordering.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• I have updated the documentation accordingly.
• I have added tests to cover my changes.