Add runner JSON schema

Problem to solve

Add missing JSON schema for the runner property inside of the Projects::JobsController#show.json response. Add the missing factory for builds factory

Further details

In gitlab-ee#7441 we need to return the runner quota information inside Projects::JobsController#show.json which was done in gitlab-ee!7233. When implementing this a new schema for the runner property was added like below which can be migrated to ce since it is usable for ce as well.

{
  "oneOf": [
    { "type": "null" },
    {
      "type": "object",
      "required": [
        "id",
        "description"
      ],
      "properties": {
        "id": { "type": "integer" },
        "description": { "type": "string" },
        "edit_path": { "type": "string" }
      }
    }
  ]
}