Include all Right Table Values in Match Column

[nbauernfeind]

Fixes #5184.

I had kept the behavior from DHE, but I suspect we do actually want for match columns to show the right values when there are no rows.

[rcaudy]

I find this "weird". I think the commingling of values is "fine", but it feels weird to me to only use the LHS name. Should we create a new name that represents a pair of values? Should we never merge values? The code change is fine, but I want confirmation from someone more user-facing.

[nbauernfeind]

This approach is being abandoned in favor of this:

• Do not include RHS columns by default (do not merge by default as is today)
• Add a union parameter that uses LHS column names and merges RHS into them (new feature)
• Allow RHS renames via the join-list (if the join-list is empty it brings all RHS columns, not jut key-columns; existing behaviors)
• Fail if joined columns have any name collisions (existing behavior)