feat: add a snap for notary

[gruyaume]

Description

Notary will not only be used in the Juju ecosystem and having a snap for it will help make notary accessible to more users. This will also be necessary for the (potential) follow-up HSM story.

This PR depends on:

• chore: change name to notary #65 being merged first
• The snap name being registered. It is now pending review because notary is a reserved name. Reference here.
• Add workflow to build and publish snap
• Add snapcraft credentials to secrets

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules