support init container/runtime container split for k8s deployment

### Enhancement Type

Improve an existing feature

### Describe the enhancement

k8s support modes of operation for a "pod" where one container acts as init container and does prep work to then have the actual containers take over running of the service

i believe it would be really nice to have that option by enabling to run either just the mod downloader/updater or to run only the server itself

this might also be a nice starting point for a mode of operation where one wants to build a image that contains the mods/datapacks to use so that downloading/plugin updating could be part of a CD pipeline while the deployment only ever would run the server

The issue names no files or tests. Start by tracing the existing container startup flow for mod downloading/updating and server launch, then determine how separate init-container and runtime-container modes should be configured for Kubernetes. Done means both modes work independently and support an image-building workflow that avoids downloading or updating during deployment.

Written by the indexing model from the issue text.