blackbox exporter charm

[lucabello]

This PR adds a first implementation of the Blackbox Exporter charm, including:

• basic charm skeleton;
• Grafana dashboard integration (with a nice dashboard to monitor the probes executed by blackbox exporter)
• Prometheus integration (self-monitoring AND periodic probes as a scrape job)
• Loki integration (through LogProxyConsumer)
• Catalogue integration (because why not)

There's some documentation in the README, but also on Charmhub Discourse:

• index
• Using the Blackbox Exporter charm

All the relations have been manually tested, and here's a screenshot of the dashboard:

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@simskij is taking care of claiming the blackbox-exporter-k8s charm name on Charmhub. If we merge the PR before that, we should run the release action again (manually) afterward, or manually publish the charm for the first time.

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[sed-i]

Great work!

Also need to:

• Update README
• Mention somewhere the existing charm?
• If you got time left for an adventure, consider unified charmcraft.yaml.

[lucabello]

About the points made in a comment above:

• README updated ✅
• I don't think it's necessary to mention that charm because it's a machine charm; sure we could link to it in the README, but I doubt it matters since this is meant to replace it
• I have the feeling the "unified charmcraft.yaml" could (e.g. would) break our CI, so I'll pass on that one 😄