feat: add pc-gadget init template

[mr-cal]

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This PR prototypes using a git submodule for the pc-gadget init profile.

If this approach works and the pc-gadget repo gets tagged releases, snapcraft can automatically update the submodule via renovate.

This can be tested for the next 30 days on the latest/edge/pr-5449 branch.

[farshidtz]

Thanks @mr-cal for prototyping this.

This works as expected, but keep in mind that there are several variants of the reference gadget snaps (pc and pi). With the submodules approach, this means one submodule for each variant.

For the pc gadget, there are a few maintained releases: https://github.com/canonical/pc-gadget/branches/all
For example, I'd image the following profile names:

• pc-gadget-20 (maybe)
• pc-gadget-22 (maybe)
• pc-gadget-24
• pc-gadget-26 (upcoming)

I think we can skip the classic gadgets.

For pi, we have to check the long term plans. If it is there to stay and not merged with pc, then:

• pi-gadget-24

Skipping the older ones which were not multi-arch snaps.

[mr-cal]

This was discussed in person, see meeting notes here.

The conclusion was that there isn't an agreement on what the init template or extension should look like. For this reason, I'm closing this prototype until there is consensus on the path forward.