Allow zstd compression for snaps

[lucyllewy]

@om26er

The store implementation that I have is currently not open-source but I will do that eventually. We are using that in our company internally.

Aah, ok, my mistake. I recall someone was working on an open snap store implementation on GitHub and had erroneously associated that detail with you

Edit: it was Futuretim: Announcing Project Kebe (open-source Snap store start) - snapd - snapcraft.io