feat: backend support for creating and storing service accounts (#22698)

Add is_service_account column to users table with CHECK constraints
enforcing login_type='none' and empty email for service accounts.
Update user creation API to validate service account constraints.

Related to:
https://linear.app/codercom/issue/PLAT-27/feat-backend-support-for-creating-and-storing-service-accounts

docs/reference/api/users.md

@@ -90,6 +90,7 @@ curl -X POST http://coder-server:8080/api/v2/users \
     "497f6eca-6276-4993-bfeb-53cbbbba6f08"
   ],
   "password": "string",
+  "service_account": true,
   "user_status": "active",
   "username": "string"
 }