Resolve "Password set emails for Users created by API"

What does this MR do?

• Extracts user creation logic from admin area controller to a service
• Adds API functionality
• Obsoletes !6637 (closed)

Why was this MR needed?

• Common feature request (#21848 (moved))
• Adds equivalent API for Webapp action

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• 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 #21848 (moved)