Add prompt when overriding a Helm-managed Flux installation

[stefanprodan]

When running flux bootstrap or flux install we should detect if Flux is already installed on the cluster with Helm.

We can look for the app.kubernetes.io/managed-by label on the gitrepositories.source.toolkit.fluxcd.io CRD. If the CRD exists and has this label, and if the value is set to Helm or any other manager besides Flux, we should prompt like so:

Flux <VERSION> has been installed on this cluster with <MANAGER>!
Are you sure you want to proceed with overriding the <MANAGER> installation? Y/N

The Flux version should be extracted from the app.kubernetes.io/version label value.

Both flux bootstrap and flux install commands should have a --force flag, when set, this will bypass the above prompt.