Add trigger information to job API

Problem to solve

Show the trigger information inside of Projects::JobController#show.json endpoint

In gitlab-org/gitlab-ce#50101 vue will start using the endpoint from

Further details

Since we are moving the job view in vue/vuex as discussed in gitlab-org/gitlab-ce#50101 vue will need to get that trigger data from a JSON api.

Proposal

Add the trigger information to the JSON response with the trigger key in build_detail_enitity.rb

Links / references

gitlab-org/gitlab-ce#50101