Added code for defining SHA attributes

What does this MR do?

This adds the ability to define SHA attributes, which are stored as binary but queries/used as if they were stored as text. This is used in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12463 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12527.

Does this MR meet the acceptance criteria?

• [x] Changelog entry added, if necessary Backend only change
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides