Add feature flags to suggestion responses

When gitlab-org/gitlab#473697 (closed) is implemented, code suggestion requests should contain information about enabled feature flags in a request header.

Parse this header and include status of these feature flags in code suggestion responses.

We already return experiments field, so we could just extend this with feature flag statuses. For example if code suggestion request contains a header:

X-Gitlab-Experiments: code_suggestions_context=1,code_suggestions_direct_completions=0

Then we would include these in the response (exp_truncate_suffix is added already by AI GW):

  "experiments": [
    {
      "name": "exp_truncate_suffix",
      "variant": 0
    },
    {
      "name": "code_suggestions_context",
      "variant": 1
    },
    {
      "name": "code_suggestions_direct_completions",
      "variant": 0
    }
  ]