Add flatpak update plugin for system-wide flatpaks

[alimirjamali]

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improves: https://github.com/QubesOS/qubes-issues/issues/2766 requires: https://github.com/QubesOS/qubes-core-agent-linux/pull/604

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 72.98%. Comparing base (b479add) to head (a22d5b2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #200   +/-   ##
=======================================
  Coverage   72.98%   72.98%           
=======================================
  Files          10       10           
  Lines        1155     1155           
=======================================
  Hits          843      843           
  Misses        312      312           

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[marmarek]

And one more thing, plugins run before normal update. It might be better to run flatpak updates after updating flatpak itself... Idk how to do that best. Either not use plugin for this (which will make progress reporting later easier), or add some option to run plugins after update (plugins_post dir?)