Send registry address with build payload

## What does this MR do?

Adds `registry_url` as a part of a build payload, when sending a response to a GitLab Runner which requested a new build.

## Are there points in the code the reviewer needs to double check?

Change in `lib/ci/api/entities.rb`.

## Why was this MR needed?

This is one of the steps needed to add support for private/protected registries hosted with GitLab CE/EE.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Related to gitlab-org/gitlab-ci-multi-runner#1828, gitlab-org/gitlab-ci-multi-runner!386, gitlab-org/gitlab-ci-multi-runner#1434

See merge request !7474