Add runner JSON schema and factory

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-org/gitlab-ee#7441 we need to return the runner quota information inside Projects::JobsController#show.json which was done in gitlab-org/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.

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

In gitlab-org/gitlab-ee!7233 the following code change to spec/factories/ci/builds.rb was made that can be ported in ce as well

diff --git a/spec/factories/ci/builds.rb b/spec/factories/ci/builds.rb
index 9813190925b..088d19fc066 100644
--- a/spec/factories/ci/builds.rb
+++ b/spec/factories/ci/builds.rb
@@ -27,6 +27,8 @@ FactoryBot.define do
 
     pipeline factory: :ci_pipeline
 
+    runner factory: :ci_runner
+
     trait :started do
       started_at 'Di 29. Okt 09:51:28 CET 2013'
     end