Gadget snap extension

[lengau]

Check existing issues

• I've verified that this request isn't described by any existing issues.

Request

Create an extension that reads the relevant values from a local gadget snap file and applies them to imagecraft.yaml.

This depends on implementing extensions.

The problem it solves

Duplication of stuff from gadget.yaml and imagecraft.yaml

[alderic-coroir]

Quoting a valuable contributor :
It's very likely that I (a customer) sell different kinds of Ubuntu Core devices, but the only real discrepancies are in the model assertion and gadget snap; I shouldn't need a new imagecraft.yaml for every SKU if I build the image the same way every time (essentially, feature-parity with ubuntu-image).