Support diff comments

[hennevogel]

Working with diff/line-based comments is not well supported in osc, so the comment type has not caught on all that much

Originally posted by @jengelh in #18537

[dmach]

This is not even supported in git-obs pr review interactive and other commands that work with Gitea where this kind of comments is supported for a long time. Rendering such comments in terminal is not straightforward.
Also, there are more critical issues in the backlog, hence giving it a low priority for now.

[hennevogel]

This is not about gitea comments, this is about OBS comments.