Confirm before overriding installation by another manager

[somtochiama]

This pull request prompts a user to confirm before overriding an existing Flux installation that's not managed by flux when running install or bootstrap.

The pull request also adds a better uninstallation prompt when flux is managed by some other tool.

Fixes: #4342

[stefanprodan]

LGTM

Thanks @somtochiama 🥇

[darkowlzz]

The overall implementation looks good to me.
Left a minor suggestion.

[darkowlzz]

I think the uninstall command can also be updated along the same lines to provide some extra information based on the cluster info. There's already a confirmation prompt for uninstall. We can just mention who is managing flux. By confirming, users will be acknowledging that although flux is managed by someone else, they want to proceed and uninstall.

[darkowlzz]

Here we know that flux is not installed at all if there's an error and it's not found error. We can print some accurate message and quit early. At present, even if it's not installed, it blindly runs the uninstall operations.

? Are you sure you want to delete Flux and its custom resource definitions? [y/N] y
► deleting toolkit.fluxcd.io finalizers in all namespaces
► deleting toolkit.fluxcd.io custom resource definitions
✗ Namespace/flux-system deletion failed: namespaces "flux-system" not found
✔ uninstall finished

Ending with status code 0 is another issue which can be addressed separately.

[somtochiama]

. Here we know that flux is not installed at all if there's an error and it's not found error.

Should we consider a case where the CRDs might not be present (maybe due to a previous incomplete uninstall) but the flux-system namespace is still present? Returning early will not delete the namespace

[stefanprodan]

We should always uninstall, this saved so many users from Helm uninstall and TF delete partial failures where some objects were stuck. I'm also not for extending this PR to uninstall, let's write an issue for improving the uninstall procedure if we must, and discuss there the specs.

[somtochiama]

I think we can keep the uninstall prompt as it doesn't change any existing uninstall behaviour and only gives the user some more information. I will open a new issue for any other changes around the uninstall procedure.

[darkowlzz]

LGTM!
Left a comment about an old in-line comment that can be updated before merging.