feat: Add snap support

[popey]

Description

This adds support for building a snap of stream-sprout. It also adds a workflow to do builds.

• Closes #22

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [X] Packaging (updates the packaging)
• [ ] Documentation (updates the documentation)

Checklist:

• [X] I have performed a self-review of my code
• [X] I have tested my code in common scenarios and confirmed there are no regressions
• [-] I have added comments to my code, particularly in hard-to-understand sections

[popey]

'2024-07-23T23:34:06.7129634Z 2024-07-23 23:34:01.096 :: 2024-07-23 23:34:00.677 Cannot pack snap: error: cannot validate snap "stream-sprout": invalid snap version ".399796c": must start with an ASCII alphanumeric (and not '.')'

The build is failing because it's running in a PR with no git tags. I suspect once this is merged, when run in an actual clone of the real repo, the version will work:-

craftctl set version=$(git describe --tags --abbrev=0).$(git rev-parse --short HEAD)

For example, if I clone the main upstream origin repo from wimpysworld, and drop the yaml in, it sets an version accordingly.

$ version=$(git describe --tags --abbrev=0).$(git rev-parse --short HEAD)
$ echo $version
0.1.4.4928a51