refactor(remote_commands): Simplify the command register method

[acelyc111]

This patch refacors on the command_manager class and related callers.

• Introduces register_single_command() and register_multiple_commands() to distinguish whether registering a single or multiple commands. The duplicate command strings in the former register_command() are reduced to avoid inconsistency.
• Changes the output of some commands to JSON format to improve readability by programs (e.g., Python scripts), and ease the coding style. (NOTE: Some output JSON strings are compacted and some are not, the compacted ones are typically simple and in one line. Including:
  • fd.allow_list
  • meta.lb.get_balance_operation_count
  • meta.lb.ignored_app_list
  • meta.lb.assign_secondary_black_list
  • engine
  • system.queue
  • server-info
  • replica.query-compact
• Improves some commands help information