Add Dingtalk Chatops Command

original MR: !417 (closed).
related MR: !474 (merged).
related issue: #532 (closed), #531 (closed).
related epic: &5300 (comment 485100).

related doc: https://drive.google.com/file/d/17D4qISvboGVGV1lxlTK9Zu2Q8ucZYJ2R/view?usp=sharing.
related video: https://drive.google.com/file/d/12i9gqwLuLLXOLJi__JYAxWUvMHQA9_gq/view?usp=sharing

1. define JH integration endpoint
2. process dingtalk message by third_party_robot_handler.rb
3. duplicate Gitlab::ChatopsCommands::Command from Gitlab::SlashCommands::Command, avoid feature break when upstream refactor.

Changelog: added