feat(workshop): add Steam Workshop downloads for Barotrauma and Stationeers

[dgibbs64]

Description

Extends the Workshop downloader (#5008, #5009, #5012) to Barotrauma and Stationeers. Neither dedicated server downloads Workshop mods itself. Both download anonymously, so no Steam account is needed.

Per-game install

• Stationeers: installs to serverfiles/mods/<id>. On start, the server adds every mod in that folder to modconfig.xml as an enabled <Local> entry, and drops entries for mods that are gone, so LinuxGSM doesn't edit modconfig.xml.
• Barotrauma: installs to LocalMods/<id>. The server only loads mods listed in config_player.xml under <regularpackages> (ContentPackageManager.RegularPackagesElementName / RegularPackagesSubElementName in the game source), so LinuxGSM keeps a <package path="LocalMods/<id>/filelist.xml" /> entry for each Workshop mod, in workshopmods order.
  • Entries for mods you added yourself are left untouched.
  • When no Workshop mods are left, the element goes back to <regularpackages />.
  • If config_player.xml doesn't exist yet (the server has never started), LinuxGSM warns and skips the step.

Testing

Live servers, running this branch:

• Stationeers (workshopmods="3813753764", Strom-Ofen electric furnace):
  • Installed to mods/3813753764, and after the restart the server wrote <Local Enabled="true"><Path Value="/data/serverfiles/mods/3813753764" /></Local> to modconfig.xml and loaded it.
  • After removal, the server removed the entry itself.
• Barotrauma (workshopmods="3815618949", Diving Suit Slot):
  • Installed to LocalMods/3815618949 and added to <regularpackages>, and the server kept the entry across restarts.
  • Removal deleted the mod and its entry.
• Both servers stayed healthy throughout and were restored afterwards.
• Locally, Barotrauma's config_player.xml editing:
  • The self-closing element is expanded, and the result stays valid XML.
  • Order follows workshopmods.
  • A hand-added entry (with its comment) is kept.
  • Removed mods' entries are dropped.
  • An empty list goes back to <regularpackages />.
• shellcheck and prettier pass.

Relates to #3514.

Type of change

• Bug fix (a change which fixes an issue).
• New feature (a change which adds functionality).
• New Server (new server added).
• Refactor (restructures existing code).
• Comment update (typo, spelling, explanation, examples, etc).

Checklist

• This pull request links to an issue.
• This pull request uses the develop branch as its base.
• This pull request subject follows the Conventional Commits standard.
• This code follows the style guidelines of this project.
• I have performed a self-review of my code.
• I have checked that this code is commented where required.
• I have provided a detailed enough description of this PR.
• I have checked if documentation needs updating.

Documentation

Covered in GameServerManagers/LinuxGSM-Docs#132.

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