Expose User API endpoint to the Front End

What does this MR do?

As per !54595 (comment 515677692)

This is needed because the current method to update user preferences through the API is for admins. We need a way for the currently logged in user to update the view_diffs_file_by_file preference through the frontend. I experimented initially with adding some kind of permissions catch in the original user endpoint but it ended up being way too messy.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #284116 (closed)