ci: add automated release and publish workflow

[Aetf]

• Enable release-plz to create release PRs automatically
• Enable automatic binary builds on Linux
• Enable version updates for rust and github actions using dependabot

Required repo secrets are

• RELEASE_PLZ_APP_ID and RELEASE_PLZ_APP_PRIVATE_KEY for the GitHub App. (See https://release-plz.ieni.dev/docs/github/token#use-a-github-app for setup)
• CARGO_REGISTRY_TOKEN for publishing to crates.io

GitHub repo further needs to be configured to allow GitHub actions to create and approve pull requets. See https://release-plz.ieni.dev/docs/github/quickstart#1-change-github-actions-permissions

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@ethanpailes we also need you to change the above repo settings and setup the workflow since I don't have admin access to the repo.

I've tested the release-plz to make sure its changelog generation still confirms to the debian changelog format. But I'll probably needs a few more iteration to tweak the workflows, since it's never possible to get them right on the first try...

Note that with this, we'll need to be a bit more disciplined with our commit messages to follow https://www.conventionalcommits.org/, otherwise the generated changelog would need to be tweaked everytime. One way to do this is to pay attention to the commit message when merging the PR with squash. This way, we can enforce a properly formatted commit message while still allow relatively effortless commits within the PR.

Fixes #53.

[Aetf]

I've setup a Github App: https://github.com/apps/release-plz-for-shpool, and added required secrets as organizational secrets.

For CARGO_REGISTRY_TOKEN, I generated a token from my crates.io account, since crates.io doesn't support team secrets.