Support discussions in `mr`
Problem to solve
As a reviewer, I often end up opening the code locally (e.g. to be able to grep around). It would help me a lot if I didn't need to switch to my browser to drop a discussion on a merge request. The current glab mr note allows for something similar, but that is not a discussion and thus it does not appear in "unresolved threads" nor does the "all threads must be resolved" merge blocker apply to it.
Proposal
The mr command should have a similar subcommand like note, but for discussions. It would be even better if it could directly support starting the discussions on certain lines as per the discussions API.
Further details
The note like interface would only be able to add a discussion to the MR in general, line specific stuff would be more complicated and I'm not sure what would be a nice interface for that. It could help if glab would basically fill out the relevant API with the hashes required and one would only need to supply the filenames and linenumbers.