Adds optional addons for microk8s

[ca-scribner]

This PR replaces #45, which itself extended #33. The PR originates from @kian99 who does not have permissions in this repo, so the full CI suite cannot be executed. I'm opening this clone of @kian99's PR so we can get the full test suite.

Copied from #45:

In order to be able to use the nginx-ingress-integrator charm and other related charms, we need to enable ingress in microk8s.

Copied from #33: This adds the microk8s-addons option, which will contain the addons to be enabled in microk8s. If not given, the default addons will be added instead (storage, dns, rbac).

This PR can be merged in favor of the original, it simply adds the original changes on top of the latest commit in main. An example use can be found here with corresponding workflow file.

Fixes #32

[ca-scribner]

45 includes a few conversations about design decisions, so reviewing the conversations there will help with understanding this PR.

[kian99]

Seems like the test failed for 2 separate reasons - the first test failed because the call microk8s status needs to be made with sudo. Then the test with the empty addons failed because part of the init script deploys coredns to the cluster and without the dns addon it fails. After deploying coredns it also deploy hostpath-provisioner and creates a service account so I suspect those would also fail without the other default addons. I'm thinking either we optionally deploy those services or add note in the readme that those addons must be enabled. Thoughts?

[ca-scribner]

oh interesting, so in (most?) cases doing addons="" is useless anyhow. darn haha. Ok let me post some fixes