feat: Adds prometheus alert rules

[saltiyazan]

Description

Adds Prometheus and Loki alert rules following spec TE071.

Initially this PR included Loki rules, but because of this bug those rules have been removed and should be addressed when the bug is fixed.

To test deploy the charm from this branch and follow the guide on how to integrate with COS, go to the alerts tabs in Grafana and then:

• To test the vault sealed rule, seal and unseal vault and observe the alert firing
• To test the leader last contact rule, reduce the threshold to something very small, maybe 5 ms and observe the alert firing
• To test the failure tolerance rule scale vault up to 3 units then back to 2 units and observe the alert firing
• I couldn't find a way to simulate leadership setup failure

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[ghislainbourgeois]

Approved. Your branch is out-of-date, please rebase or merge main on it before merging.