Duo Workflow direct access endpoint

What does this MR do and why?

This MR implements item 1 in this authentication architecture diagram.

For this, this MR adds a new endpoint POST /api/v4/ai/duo_workflows/direct_access

The endpoint returns following payload

{
  "gitlab_rails": {
    "base_url": "http://gdk.test:3000",
    "token": [an oauth token]
  },
  "duo_workflow_service": {
    "base_url": [DUO_WORKFLOW_SERVICE_URL env var],
    "token": [a duo workflow service token],
    "headers": {
      "X-Gitlab-Host-Name": "gdk.test",
      "X-Gitlab-Instance-Id": "12f5283d-2102-411f-b87e-5d03f33ad141",
      "X-Gitlab-Realm": "saas",
      "X-Gitlab-Version": "17.2.0",
      "X-Gitlab-Global-User-Id": "HlZcbHK5Wa5s07e8+Bp5eCxWIDpsF+t0haQK0+14R0g="
    }
  }
}

The gitlab_rails.token and duo_workflow_service.token are returning "not implemented" and will be handled in a separate issues: #471898 (closed) #470862 (closed)

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Before	After

How to set up and validate locally

1. Set up local duo-workflow-service following docs
2. If you are not running GitLab Rails on localhost:3000, update DUO_WORKFLOW_AUTH__OIDC_GITLAB_URL value in the .env file on your Duo Workflow Service
3. Update .env in the Duo Workflow service so that DUO_WORKFLOW_AUTH__ENABLED=true
4. Run Duo Workflow Service server (poetry run python -m duo_workflow_service.server)
5. Set DUO_WORKFLOW_SERVICE_URL in env.runit to localhost:50052 (where the duo workflow service is running) and gdk restart rails.
6. curl -XPOST http://gdk.test:3000/api/v4/ai/duo_workflows/direct_access\?private_token\=[your_token]
7. You should receive a success response similar to the payload given above.

Related to #470128 (closed)