T7827: Add unionfs-fuse package

[sarthurdev]

Change summary

Build our own unionfs-fuse package. Package not present in Trixie.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

Related PR(s)

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

👍
No issues in PR Title / Commit Title

[sever-sever]

add build unionfs-fuse

[dmbaturin]

For context: unionfs-fuse is required for the legacy config backend that represents the running configuration and proposed configuration from user sessions as directories mounted via UnionFS, and it relies on subtle behavior details of UnionFS that make migration to OverlayFS or anything else very difficult, so we have long decided to just keep using UnionFS via FUSE until we completely replace the legacy config backend.

We aren't required to build this package from source yet, but it's removed from Debian Trixie, so when the time comes to update to Trixie we will need to start building it (assuming we don't get rid of the legacy configuration backend by then).

I think we should start building it now, we can always just remove this build script when the package is no longer needed.