Add application create API

What does this MR do?

Adding an API for creating applications

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

This is my first MR, i'm not sure about your good practices. Also, i'm not native English speaker.

Why was this MR needed?

It resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/24035

Screenshots (if relevant)

Not relevant

Does this MR meet the acceptance criteria?

• [ ✓] Changelog entry added
• [✓ ] Documentation created/updated
• [ ✓] API support added
• Tests
  • [ ✓] Added for this feature/bug
  • [× , not my code?] 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?

Closes #24035 (moved)