Use feature definitions for experiments api

What does this MR do?

Closes #324568 (closed)

We switch the experiments API to use the Feature::Definitions, since I believe this is now a constant concept between implementations.

Screenshots (strongly suggested)

Old:

{
  "key": "experiment_1",
  "enabled": true
}

New:

{
  "key": "null_hypothesis",
  "state": "off",
  "enabled": false,
  "name": "null_hypothesis",
  "introduced_by_url": "https://gitlab.com/gitlab-org/gitlab/-/merge_requests/45840",
  "rollout_issue_url": null,
  "milestone": "13.7",
  "type": "experiment",
  "group": "group::adoption",
  "default_enabled": false
}

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