feat: release helm charts to grafana/helm-charts

[theSuess]

Implements #1732

Uses the same approach as the k6-operator to publish releases to the grafana/helm-charts repo

[theSuess]

We might want to split this into a separate task to allow for faster iteration without triggering the complete release flow

[weisdd]

I'm not entirely sure if I correctly understood the workflow, but seems like, amongst other artifacts, it'll push an OCI artifact to the same repository as we do now, which means we'll push it twice (through helm-oci and helm-charts-repo). What's also important is that the new workflow relies on helm v3.5.2 (was released in Feb 2021), whereas support for OCI graduated to GA only in v3.8.0 (e.g. prior to v3.7.0, helm used a different layer media type), so the artifacts would not be the same. I'm not sure if it'll break anything, but it's certainly not a good practice to use such an old helm version :)

UPD: I see that helm version has got updated in the workflow in the helm-charts repository a few hours ago, so helm version is not a concern anymore, but the fact that we would attempt to push to the same repository - is (again, assuming I've correctly understood the workflow).

[CarlosLanderas]

Thanks for this @theSuess . I was suffering the problem you mention in the open issue and need to install grafana operator from non OCI source.

[theSuess]

Good call @weisdd! I switched from just importing the chart to just pushing the release from our release flow as I also realized that the change detection doesn't work for our use case