Improved support for per-arch runtime extension inheritance

[nanonyme]

flatpak-builder has support for inherit-sdk-extension. This is though all or nothing which is creating issues in https://invent.kde.org/packaging/flatpak-kde-runtime/-/issues/15 where the extension contains architecture name to support a multi-arch scenario. It would be good if through some machinery we could still use the simplicity of inherit-sdk-extension but not break on architectures where the extension does not exist.

[nanonyme]

Ping @alexlarsson @refi64 any thoughts? Some suggested options:

• explicit per-arch inheritance
• ignore inherited extension if missing from parent runtime
• not solving here but adding fake extensions to freedesktop-sdk so flatpak-builder will not break

[theofficialgman]

anyone willing to work on this? currenty a blocker for adding additional architecture support on multiple flatpaks

[orowith2os]

Would it be possible to use only-arches flags for this use case? We can already do this for sources, it seems appropriate to add the ability to extensions.