Merge branch 'feature/expose-timestimes-in-build-entity' into 'master'

Expose timestamps in build entity

## What does this MR do?

This MR exposes timestamps for build entity that is being used by serializers in `app/serializers`. 

## Does this MR meet the acceptance criteria?

- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing

## What are the relevant issue numbers?

See https://gitlab.com/gitlab-org/gitlab-ce/issues/24844#note_19145053

See merge request !7818