Add plugin gs-plugin-systemd-sysupdate

Create a new plugin gs-plugin-systemd-sysupdate to support OS upgrade through systemd-sysupdated service

Important: This MR is based on the systemd-sysupdate MR, which implements the sysupdated service for high layer to access systemd-sysupdate through D-Bus. That is, this plugin works only after the above systemd MR has been merged and released.

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This plugin implements the plugin interface and the functions to

• discover new apps by querying update targets (host as OS upgrade and component app updates) from sysupdated
• manage the apps created and perform actions update() and cancel() through sysupdated

This plugin current does NOT adopt any apps

Note: Based on the sysupdated design, this plugin should work with AppStream. However, this might need to introduce new AS_KIND and AS_BUNDLE to the AppStream spec. and also require the support in libappstream and gnome-software. All these things are out of the scope in this MR.

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Verification done:

• run self test with the command $ meson test gs-self-test-systemd-sysupdate -v, which tests:
  1. create apps from targets
  2. search apps with specific keywords 'sysupdate'
  3. run app upgrade/updates with progress update
  4. cancel app upgrade/updates
  5. update and remove target stored within plugin
• test manually using gnome-os with sysupdated patch to test with local update repo.
  1. trigger refresh to check apps are created for targets
  2. trigger, check download progress and cancel app upgrade/updates
  3. trigger and wait app upgrade/updates success