Accept image for avatar in user API

What does this MR do?

Add ability to upload avatar image through user API

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

In issue was proposed only to change through admin users API, but in comment to change users own avatars. I don't find APIs to change user own properties. To my mind, we should do it in separate issue. Also, we need to rewrite users API doc to ones with tables, like project (?)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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?

Closes #26212 (closed)