API to get allowed agents by CI_JOB_TOKEN

New api endpoint: /api/v4/job/allowed_agents

returns:

{
  "allowed_agents": [
    {
        "id": 5,  // agent id
        "config_project": {
          "id": 3,
        }
    },
    {
        "id": 3,
        "config_project": {
          "id": 3,
        }
    },
  ],
  "job": {
    "id": 9 // job id
  },
  "project": {
    "id": 123213 // project id
  },
  "pipeline": {
    "id": 2323 // pipeline id
  },
  "user": {
    "id": 1,
    "username": "root"
  }
}

Specifications from the epic:

• allowed_agents will only include agents for the JOB_TOKEN project. Adding external agents requires additional permissions, which will be handled in follow-up issues.
• Endpoint auth: The user running the CI job should have Developer rights on the project configuring the agent used.
• This feature can be disabled -- I think this will be handled in the agentk config file so the API does not need to worry about this setting.