build-aux: Add versions to Flatpak plugin extension point

[tytan652]

Description

"version" and "branch" are kind of the same in the context of extension point.

• Enables access to stable branch (Flathub) plugin to Flathub Beta releases
• Strip away access to beta branch plugins (Flathub Beta), mostly unused by submitters
• Enables test Flatpak plugin from Flathub submissions to be tested

Before:

• Flathub release (stable) list only plugin from Flathub (stable)
• Flathub Beta release (beta) list only plugin from Flathub Beta (beta)

After:

• Flathub release (stable) list only plugin from Flathub (stable), and allow test branch plugins
• Flathub Beta release (beta) list only plugin from Flathub (stable), and allow test branch plugins

Motivation and Context

• Enable access to all stable plugins to OBS Studio beta releases
• Ease plugin submission testing

How Has This Been Tested?

Local build+install (Flatpak branch set to master), stable branch plugin are still loaded (and test branch is loaded).

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.