Add snap packaging

[artivis]

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What

This PR adds snap packaging for the CLI/GUI together with a CI job to automatically build and release the snap.

The snap is already available on the store but isn't publicly listed yet. One can give it a try with:

snap install rerun --candidate

to try a 0.20.3 based build or,

snap install rerun --beta

for a build based on main (at 9699349332d27010e37edcc67b164530e13a07e7).

The Rerun's examples work fine both on an Intel integrated and Nvidia discrete GPUs on an amd64 arch. However more testing/feedback is required to make sure all features are working fine.

I currently own the name rerun on the store but I'd be more than happy to transfer it to the project maintainers if they decide to take on the support for this snap. I can also help in getting started 👍 .

Regarding the CI, it is currently a separate workflow that builds and tests (simply runs rerun help) the snap on PRs and pushes to main. It also publishes the snap on the store (once a Store login token is provided as a secret). It publishes the snap on the beta track for any push on main and on the candidate track on tags. Promoting the snap to the stable track is expected to be done manually after further testing.

I hope this is interesting to the maintainers and the community at large!

Please do feel free to hit me with any question/feedback.