Implement Users::BuildService

What does this MR do?

Implements a new service for building users.

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

The duplicate user.generate_reset_token in both Users::CreateService and Users::BuildService.

Why was this MR needed?

To make code more readable and replace Users::CreateService.new(params).build which is a bit confusing.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] 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 #30349 (closed)