Implement LFS locking API

What does this MR do?

Implements the Git LFS File Locking API according to the latest stable release.

Additionally implements an integration with the current File Locking feature.

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

Why was this MR needed?

Because we were lacking support for Git LFS File Locking

Screenshots (if relevant)

A small sample of the Git client using the File Locking API

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #1830 (closed)