Add support for unsup

### Enhancement Type

New server type

### Describe the enhancement

[unsup](https://git.sleeping.town/unascribed/unsup) (Una's Simple Updater) is an extension to the packwiz format which allows upgrading of the game components and supports pre-defined variants of a pack. It fills a similar niche to packwiz, but supports more exotic game configurations.

While this jar can be run as a java agent, it is unable to perform a 'cold start' given the Docker image needs a pre-existing sever jar.

The 'flagship' pack using unsup is [Rewind Upsilon](https://git.sleeping.town/Rewind/Upsilon) (a touchup of FTB Ultimate) and a reference usage can be seen in their [server files](https://rewindmc.com/dl/upsilon-server.zip) (will download a `.zip`).

Start by reviewing the Docker image's startup behavior and the linked unsup and Rewind Upsilon server files, focusing on the pre-existing server-jar requirement and cold-start flow. The issue names no repository files or tests; done would mean documenting and validating support for an unsup-based server configuration.

Written by the indexing model from the issue text.