Epic: Finish off merge insert functionality

[westonpace]

We have some basic merge insert functionality in lance today. Other databases have more sophisticated capabilities. Perhaps the most extensive and well documented is the "merge into" from SQL server. This issue will keep track of the various things that aren't yet implemented in our version.

• [x] #2610
• [ ] Allow update columns to be projection statements This will allow for things like `when_matched().update({"count": "source.count + target.count"})
• [ ] Allow updates to be applied to "when not matched by source" rows This allows for updating target rows that don't match anything in the source (today we can just delete them)
• [ ] https://github.com/lancedb/lance/issues/1983
• [ ] https://github.com/lancedb/lance/issues/2621

[mutecamel]

merge_insert is one of the reasons I switch to Lance and LanceDB. Thank you for your hard work!