text: Title: Small refactoring: make _custom_command an instance method of CustomParser
  Body: We have a `_custom_command` function: https://github.com/ploomber/ploomber/blob/2e0d764c8f914ba21480b275c545ea50ff800513/src/ploomber/cli/parsers.py#L399
  
  that should really be an instance method of `CustomParser`: https://github.com/ploomber/ploomber/blob/2e0d764c8f914ba21480b275c545ea50ff800513/src/ploomber/cli/parsers.py#L36
  
  Steps:
  
  - [ ] make _custom_command an instance method
  - [ ] refactor all tests that call _custom_command
  - [ ] rename _custom_command to `load_from_entry_point_arg`
  