Add the ability to slack API to query saved messages

[hendrysadrak]

Description

I would like to have the opportunity for reflecting saved messages in another todo tool. When I save a message, I want a todo item to be created, and when unsaving, it to be deleted/completed.

This would mean adding one or both of the following solutions:

• Ability for querying the saved messages with the Slack API
• Have events for 'message saved' and 'message unsaved' in Events API

What type of issue is this?

• [ ] bug
• [x] enhancement (feature request)
• [ ] question
• [ ] documentation related
• [ ] testing related
• [ ] discussion

Requirements

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've searched for any related issues and avoided creating a duplicate issue.