build(release): add linux .deb and .tar.xz packaging

[devroy10]

What changed

Adds Linux packaging for the DroidCam OBS plugin by repackaging the official
Linux release zip:

• A GitHub Actions workflow (.github/workflows/build-linux-packages.yml) that
  runs when a release is published (or manually, with a version input),
  downloads the official droidcam_obs_<version>_linux_x86_64.zip from the
  release assets, and attaches two artifacts to the release:
  • droidcam-obs_<version>_amd64.deb: installs the plugin into the system
    OBS plugin directories (binary in /usr/lib/x86_64-linux-gnu/obs-plugins,
    data in /usr/share/obs/obs-plugins/droidcam-obs), with minimal stable
    dependencies (obs-studio >= 30.0, libstdc++6).
  • droidcam-obs-<version>-linux-x86_64.tar.xz: mirrors the zip layout plus
    install.sh, README.TXT and LICENSE.
• packaging/build-linux.sh and packaging/debian/ metadata.
• packaging/README.md documenting the flow.

Why

Linux users currently get a zip they unpack by hand. A .deb gives Debian and
Ubuntu users native installation, and the .tar.xz follows common packaging
conventions. Repackaging the official binary (instead of compiling from
source) keeps the workflow robust across releases: no library package name
pinning, no linker flags, and the artifact is exactly what the maintainers
tested. If the zip is renamed or dropped, the workflow fails loudly at the
download step.

Impact

Debian/Ubuntu users can install with sudo apt install ./droidcam-obs_*.deb.
There is no change to the plugin itself.

Validation

• Packaged the official 2.5.1 zip locally and inspected the resulting .deb
  and .tar.xz.
• Installed the .deb on Ubuntu 24.04 with OBS 32.2 and confirmed OBS loads
  the plugin: [droidcam-obs] module loaded release 251.
• Extracted the .tar.xz into the user plugins directory and confirmed OBS
  loads the plugin the same way, including a live scene streaming a phone
  over ADB.