[V1] Replace PlanTest equivalence with Ion rep.

[RCHowell]

Description

This issue is to track replacement of the PlanTest equivalence function (and pretty-printing) to use the Ion serde representation of plans. It currently uses the 0.14 pretty-print combined with data classes to get an equals/hashcode.

Now that the plan is a set of interfaces, we'll need a mechanism for comparing plans .. likely to be similar to the basic equivalence visitor.

https://github.com/partiql/partiql-lang-kotlin/blob/main/partiql-planner/src/test/kotlin/org/partiql/planner/util/PlanNodeEquivalentVisitor.kt#L16