Create an internal feature API to evaluate ff (one at a time)

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Context

Following from discussion in Investigate a path forward to use "Gitlab Featu... (container-registry#1104 - closed) :

We need to create an internal "Features" API That can be used to evaluate existing Gitlab development feature flags from gitlab sub components like the container registry.

Currently the Feature flags API exists but it does not provide:

• The ability to evaluate a specific feature flag
• The ability to authorize a gitlab subcomponent via a shared secret with the subcomponent

Requirements

We want to create a new internal API endpoint to allow evaluating a single feature defined in rails, this new endpoint will be gated by shared secrets issued by Gitlab rails:

RequestL GET internal/features/{{feature_name}}?service={{subcomponent_name}} Response: 200 OK

  {
    "name": "{{feature_name}}",
    "state": "on",
    "gates": [
      {
        "key": "percentage_of_actors",
        "value": 34
      }
    ],
    "definition": {
      "name": "{{feature_name}}",
      "introduced_by_url": "https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40880",
      "rollout_issue_url": "https://gitlab.com/gitlab-org/gitlab/-/issues/244905",
      "group": "group::ci",
      "type": "development",
      "default_enabled": false
    }
  }