feat: IAM-286 preliminary commit for charm base

[wood-push-melon]

What

This pull request aims to provide the preliminary version of the GLAuth base charm.

I'm sorry this pull request is chunky since it's based on the code used in the demo. Please focus on the code under src when reviewing.

What's inside

• Basic juju event handling
• Postgresql charm relation
• Kubernetes ConfigMap for GLAuth configuration
• Integration with COS (sorry, this is because the code is based on the demo)

Improvements on the way

• Kubernetes client and resource improvements for more extensibility
• Flaky scenario: remove integration and re-integrate with Postgresql charm (due to the varying delay mentioned in https://kubernetes.io/docs/concepts/configuration/configmap/#mounted-configmaps-are-updated-automatically). Already have an idea to improve this issue, and going to improve it in another pull request
• Pre-commit enforcement

[nsklikas]

Overall looks good to me, but I have some comments. I suppose we are going to rework the kubernetes_resource logic either way (we should use the same logic in all of our charms).

[nsklikas]

One final comment. Also a lot of functions are missing return type annotations (and there are some other typing errors). Running mypy should show you whatever issues there are. It does not make sense to fix some of them, but let's try to resolve as many as possible.