[V1] Post rc.3 plan cleanup follow-up issue

[RCHowell]

Description

This issue holds a list of follow-up actions for post rc.3 plan development.

Tasks

• Collation getColumn should return a column index rather than an expression.
• RelAggregate group strategy
• Document all arg ordering (import because of efficient serde and rewriter rules/strategies!)
• [V1] Re-evaluate temporary left and right schemas from the logical join operators. #1575
• Planner subquery support (proper)