ci: promote snap to candidate on every release

[cjdcordeiro]

closes #241

BEFORE MERGING: This new job requires a secret for letting Snapcraft authenticate with the Snap Store. I'm still in the process of getting a Pebble-specific bot account for this, so before merging, please double-check the current status with me so that I can also share said secret privately.

Context

This PR is built on top of #179, allowing us to automatically promote the Pebble snap to latest/candidate every time there is a new Pebble release on GitHub.

Summary of changes

• this workflow is now also triggered on every GH release
• during the snap tests, the Pebble client version is inferred so that it can be used for promoting the right snap version
• on release, we wait for the Store to finish the snap builds for every architecture
• once the store has built and released the respective snap into latest/edge, we then promote it to latest/candidate

---

Example of a successful build: https://github.com/cjdcordeiro/pebble/actions/runs/5475322209

[benhoyt]

This looks okay to me, though I'm not the best person to review it (I don't have much knowledge of snapcraft and what's possible or not possible with it).

[cjdcordeiro]

@benhoyt @jnsgruk @niemeyer

Update:

We now have a bot account that we can use from within the GitHub workflows to login with the Snap store and promote the Pebble snap.

I have already shared the credentials with you.

Please add the SNAPCRAFT_STORE_CREDENTIALS secret to this repo and then the PR can be considered to be merged.

[jnsgruk]

Please add the SNAPCRAFT_STORE_CREDENTIALS secret to this repo and then the PR can be considered to be merged.

Done!