opt, sql: support PlanGram matching in FK cascades

[michae2]

FK cascades are planned and run after the main query as postqueries. We should provide a way to control FK cascade plans using PlanGram matching.

Some ideas on how to implement this:

• we could add a dummy expression to the PlanGram language to indicate which part of the PlanGram is a FK cascade
• or have a separate statement hint type for FK cascades
• or add another field to the pin plan statement hint protobuf for FK cascade PlanGram(s)

Jira issue: CRDB-65487

[michae2]

Note that when working on this we'll want to be sure that recursive PlanGrams work for cyclical FK cascades.