Add possibility to return only charts compatible with running cluster

Currently Helm chart proxy endpoint returns in index.yaml all charts available in the original repo, independently if they are compatible with the running cluster (e.g. k8s version) or not.

Helm chart can specify what are k8s version it can be applied.

Acceptance criteria

• /api/helm/charts/index.yaml accepts onlyCompatible flag (default: false)
• If true, it returns index.yaml containing only charts compatible with the running clusters. Charts not specifying k8s version are considered compatible