Custom startup/shutdown commands

### Enhancement Type

A completely new feature

### Describe the enhancement

It'd be nice to be able to run some system commands when the server has started or is shutting down. For example sending a message on a discord webhook without having to install a mod.

Start by tracing the image's server startup and shutdown entry points and how lifecycle processes are managed. Define how users would configure commands, when each command runs, and what happens on failure; done means configured system commands run reliably at both lifecycle events without requiring a mod.

Written by the indexing model from the issue text.