Introduce `Environment` kind CRD for grouping resources per environment

Release notes

Problem to solve

• As an Application Operator, I want to see what is currently running in my namespaces/clusters.
• As an Application Operator, I want to control and monitor rollouts.
• As a Release Manager, I want to control and monitor rollouts.

Add Environment support to deployments managed by the GitLab Agent for Kubernetes.

Proposal

By @marshall007:

I think a more logical short-term fit for the Agent is for it to facilitate the creation and teardown of "environments" within GitLab via the Kubernetes API (in the form of CRDs and/or annotations on the ConfigMap inventory object). In other words, let the cluster be the authoritative source of "environments" and "deployments" and have the Agent act as a shim for syncing that cluster state back to GitLab.

This would be awesome because users would no longer need to explicitly implement a CI job for tearing down environments. Tearing down an environment becomes synonymous with pruning all the resources associated with an inventory object/id. This could now be done either via the GitLab UI or manually in the cluster to the same effect. Currently if you manually delete the namespace in the cluster associated with a review app, GitLab is unaware that this environment has been deleted.

A ton of discussion happened in #350697 (comment 833047627)

apiVersion: k8s.gitlab.io/v1
kind: Environment
metadata:
  name: review-023f1bce01-my-review-branch
spec:
  name: review/my-review-branch
  externalUrl: https://023f1bce01-my-review-branch.review.example.com
  autoStop: "1 week"
  scope:
    inventoryId: 01243567789-my-inventory-id
    labels:
      customLabel: customValue
    namespace: my-environment-namespace

The UI could show the Kubernetes resources returned based on the scope filters:

• inventoryId: return the resources owned by the given inventory id and their siblings

• labels: return the resources that match the given labels

• namespace: return the resources from the given namespace only

• these filters are AND-ed together

• The list of filters can be extended later if needed.

• An Environment with an inventoryId filter could be auto-created by specifying the other parts of spec as part of the gitops.manifest_projects configuration to simplify the integration setup.

Intended users

• Rachel (Release Manager)
• Allison (Application Ops)

Metrics

• Any ideas for MAU based metrics?

• Number of environments updated / month

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