snap packages: Support passing arguments to snapcraft

[Maximilian Federle]

For various reasons, building snaps might require passing additional arguments to snapcraft.
A common use case: Passing --enable-experimental-extensions to be able to use the kde-neon extension on core20.
It would be nice if it were possible to configure such arguments.

Bonus objective: Enable the same for snapcraft's remote-build feature. The corresponding bug is here: https://bugs.launchpad.net/snapcraft/+bug/1950867

[Talha Can Havadar (tchavadar)]

Recently I also needed to use --enable-experimental-plugins to use kernel plugin with snapcraft

[Colin Watson (cjwatson)]

I think this should be done using individual structured database fields for particular flags such as "enable experimental extensions", rather than a general mechanism to add arbitrary options.