Auto detect gitlab-managed-apps in cluster
### Description
We currently are try to get up and running with the kubernetes integration. Adding the same cluster to multiple projects first is painstaking annoying and clumbsy, some of the issues we are facing should be solved by `Group Clusters`. The lack of ability to control `Clusters` via API (or atleast documentation of those API endpoints) makes this a lot of manual labor. Given that each service we run have their own project in Gitlab.
We do however want to run our own prometheus instance and NGINX ingress, as we need to have more control over parts of the deployments, and can't find a way to supply extra values.yaml or other deploy-time configuration options.
### Proposal
1. When adding cluster to project (or group), check for already existing `gitlab-managed-apps` namespace
2. Verify the deployment status for any available "one-click installs" available from Gitlab
#### Nice to have
If a service is not installed in `ns/gitlab-managed-apps` allow the user to specify a service endpoint. The user might have several instances of a given "core" service running.
##### Example:
We run 1 cluster, but have separate namespaces and Prometheus instances for monitoring test and production environments.
### Links / references
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