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?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

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