Add internal API to create access token for suggested reviewers

What does this MR do and why?

This MR adds a new internal endpoint for suggested reviewers to get short-lived access tokens and query merge requests via the public API (GraphQL).

• This API is gated behind a feature flag suggested_reviewers_control.
• Documentation change to the Internal API page will be in a separate MR.

• change in this MR is highlighted in #f97f71

Implementations

POST /internal/suggested_reviewers/tokens

Attribute	Type	Required	Description
project_id	Interger	Yes	The ID of the project

The access token is:

• Generated under suggested_reviewers_bot user
• Valid for 1 day (we can't go any lower than that given the expires_at only support date granularity)
• With access level Reporter
• With scope read_api

Example request:

curl --request POST "https://gitlab.example.com/api/v4/internal/suggested_reviewers/tokens" \
  --header "Gitlab-Sugggested-Reviewers-Api-Request: <jwt_token>" \
  --header "Content-type: application/json" \
  --data '{ "project_id": <project_id> }'

Example response:

{
  "id": 74,
  "name": "Suggested reviewers token",
  "revoked": false,
  "created_at": "2022-11-15T10:11:56.258Z",
  "scopes": [
    "read_api"
  ],
  "user_id": 129,
  "last_used_at": null,
  "active": false,
  "expires_at": "2022-11-15",
  "access_level": 20,
  "token": "glpat-<enacted>"
}

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. Ensure a SaaS (Gitlab.com) environment
   1. One way of doing this is to add a env.runit file to the root GDK folder with the following snippet

      export GITLAB_SIMULATE_SAAS=1

2. Set ultimate license on a group http://gdk.test:3000/admin/groups
3. Create a project in the ultimate group or use an existing one, e.g. http://gdk.test:3000/gitlab-org/gitlab-test
4. Set the feature flag on rails console bundle exec rails c

   project = Project.find(2)
   Feature.enable(:suggested_reviewers_control, project)

5. Enable suggested_reviewers_enabled project settings

   project.project_setting.update!(suggested_reviewers_enabled: true)

6. Get the secret from the console

   secret = Gitlab::AppliedMl::SuggestedReviewers.secret

7. Generate a JWT Token

   jwt_token = JWT.encode(
     { 'iss' => Gitlab::AppliedMl::SuggestedReviewers::JWT_ISSUER, 'iat' => 1.minute.ago.to_i },
     secret, 'HS256'
   )

8. Execute a cURL request to create a new access token for the above project

   curl --request POST \
     --url http://gdk.test:3000/api/v4/internal/suggested_reviewers/tokens \
     --header 'Content-Type: application/json' \
     --header 'Gitlab-Sugggested-Reviewers-Api-Request: <jwt_token>' \
     --data '{ "project_id": 1 }'

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #379635