Unify index key representation in query plan

[joshua-spacetime]

Description of Changes

Refactors index probes so that multi-column index keys are represented as a single product value expression PhysicalExpr::Product instead of being split across various fields and structs. As a result, this patch also simplifies the index-scan and index-join query executor variants now that index scans/probes share one physical shape.

This change is in preparation for adding formal parameters to query plans. Since index probe values now have single unified representation as a PhysicalExpr, a future parameterized plan can represent an index scan or index join as follows:

IndexProbe::Point(PhysicalExpr::Product(vec![
    PhysicalExpr::Param(sender_slot),
    PhysicalExpr::Value(other_const),
]))

PhysicalExpr::Product(vec![
    PhysicalExpr::Param(sender_slot),
    PhysicalExpr::Field(lhs_join_field),
])

This will avoid having to duplicate parameter handling in a bunch of different places or rules.

Note, a very nice consequence of this refactor is that it removes/consolidates a significant amount of code as the diff shows.

API and ABI breaking changes

N/A

Expected complexity level and risk

...

Testing

Existing coverage