Add `appstream://` URI handler

[aleasto]

Is there an existing issue for this?

• I have searched the existing issues and found none matching what I'm reporting.

Is the feature related to a problem or existing issue?

No response

Describe the feature, why the feature matters to you, and what benefits the feature will bring about.

Since we do support package discovery via appstream from within the app, it would be nice to also support opening appstream:// URIs, which are usually provided by online stores like https://apps.kde.org.
This would be equivalent to opening a snap:// URI.

Describe any alternatives you've considered.

No response

Additional context

No response