Improve SQL planner & logical plan support for JOIN conditions

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
The logical plan for joins currently only supports equijoins on columns and does not support joining on other expression types:

    /// Equijoin clause expressed as pairs of (left, right) join columns
    pub on: Vec<(Column, Column)>,

Describe the solution you'd like
I would the SQL planner and logical plan to be able to model a wider range of join conditions. At physical planning time, we would still need to fail for now if the join is not an equijoin on columns.

Describe alternatives you've considered
None

Additional context
None