Expose Appstream's icon policy options

[bbhtt]

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

This was raised as a PR first in #767 but I closed it as it was missing discussion on why it was needed, the PR contents were not upto standard and parts of the PR were blocked on porting to libappstream API (still is).

I need some explainer on why the icon policy needs to be exposed in the first place as in why the default sizes are not sufficient.

My understanding from Flathub times was that there was a need for larger and larger sized icons for HiDPI/hero banner etc. but that also causes bandwidth issues.

I remember also trying out the icon policy argument when it was first added to Appstream and it didn't work as expected.

I wonder if it would be still needed after Appstream exposes JXL media.

[bbhtt]

Also I want it done after API port as done so that we can properly put in a minimum version req at build time, instead of dirty runtime checks.

This removes the whole problem of asking for endless backwards compat as we can (conservatively) raise the version requirements.

We will need 1.2.0 for the port anyways so it should support all modern features.