More intelligent default Sdk extension version

[hfiguiere]

Checklist

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

flatpak-builder version

1.2.2

Flatpak version

1.12.7

How to reproduce

1. Use GNOME or KDE SDK
2. Add and extension to the manifest (rust-stable will work). Make sure it is NOT installed
3. Run flatpak-builder

Expected Behavior

It offer to install the matching SDK extension, guessing the proper version, GNOME 43 -> version 22.08

Actual Behavior

It tells me it cant find the SDK extension version 43

Additional Information

The version is in the extension point of the SDK/Runtime.

#469 show we can't specify the SDK extension with a tuple (id//version) that could be an ok workaround

If the right SDK extension is already installed there is no problem.

[TingPing]

This just sounds like a new feature. It doesn't really "guess" anything. It just defaults to the same version as the runtime.

[hfiguiere]

From a user standpoint it's a bug.

I don't know how feasible that is, but the determination of the version of the extension can be made by checking the extension point for the version property.

[Justinzobel]

A possible option (from my perspective) would be to have a version specified in the manifest:

    "sdk-extensions": [
        "org.freedesktop.Sdk.Extension.rust-stable//23.08"
    ],

[hfiguiere]

A possible option (from my perspective) would be to have a version specified in the manifest:

    "sdk-extensions": [
        "org.freedesktop.Sdk.Extension.rust-stable//23.08"
    ],

See #469