Kubernetes deployments on new clusters will now have
a separate namespace per project environment, instead
of sharing a single namespace for the project.

Behaviour of existing clusters is unchanged.

All new functionality is controlled by the
:kubernetes_namespace_per_environment feature flag,
which is safe to enable/disable at any time.

- Closes #60024

- Change PrometheusClient.new to accept a base url instead of an
  already created RestClient

- Use Gitlab::HTTP in PrometheusClient instead of creating RestClient
  in PrometheusService

- Move http_options from PrometheusService to
  PrometheusClient (follow_redirects: false)

- ensure that base urls don't have the trailing slash

- Created a `PrometheusClient#url` method that might not be strictly
  required

- Change rescued exceptions from RestClient::* to
  HTTParty::ResponseError where possible and StandardError for the
  rest

Our current version of Cert-Manager does not uninstall
cleanly, and we must manually remove custom resource
definitions.

Also creates specs

Only allow Helm to be uninstalled if it's the only app

- Remove Tiller leftovers after reser command
- Fixes specs and offenses

Adds changelog file

Fix reset_command specs

- After uninstalling the knative helm chart it's necessary to also
remove some leftover resources to allow the cluster to be clean
and knative to be reinstalleable.
- Adds knative uninstall disclaimer
- Uninstall ksvc before uninstalling knative

Make list of Knative and Ingres resources explicit

- To avoid deleting unwanted resources we are listing exact
which resources will be deleted rather than simply deleting any
resource that contains istio or knative words.

All cluster resources are now created on demand when
a deployment job starts.

- Set as uninstallable app
- Update docs
- Adjust specs

- This enables us to use a scope to query all clusters in group
hierarchy order in one query, and also enables us to union to instance
clusters later.

- Handle case where clusters not present at level. In which case the
query should go ahead and return the next level's clusters.

- Swap with new CTE query behind Feature flag. This FF is default
disabled.