[Feature request]: Allow services to be restarted when extension is installed

[eeejay]

Checklist

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Suggestion

A flatpak may export a service that relies on extensions for functionality. The service will not have access to a newly installed extension if it is already running because the extension's files will not be mounted into an existing sandbox.

I propose that a service can export an interface that would allow the host to respawn the service after an extension is installed. The interface will have a Shutdown() method, similar to my proposal in issue #5775, and a Spawn() method. The Shutdown() one would be called, followed by the Spawn()

Note

I understand that there is no obvious need for a Spawn() method because the whole point of activatable services is that they will become alive when called. The case where this is needed is if there are already open apps that have a proxy to this service. Unless it is re-spawned, any changed D-Bus properties will not get updated in the proxy.