Ability to list and edit comments (discussion) using az boards work-item xxx commands

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is. Ex. I cannot create a project..

**Describe the solution you'd like**
Ability to edit discussion items for a work item.

Currently I can only append comments to a work item using:
`az boards work-item update --id $issue.id --discussion $newBody`

A clear and concise description of what you want to happen.

something following the standard convention would be nice like

az boards work-item discussion show --work-item-id 123 -> list all discussion items for work item 123
az boards work-item discussion show --id xxx -> show comment id xxx
az boards work-item discussion delete --id xxx -> delete comment id xxx
az boards work-item discussion update --id xxx -> update comment id xxx

**Additional context**
Add any other context or screenshots about the feature request here.
Also the ability to modify/specifiy the user that made the comment( usefull for automation or migrations)