opt, sql: support PlanGram matching in FK cascades

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

Epic: CRDB-66266

Start by tracing the existing PlanGram matching and pin plan statement hint handling for FK cascade postqueries. Compare the proposed dummy expression, separate hint type, and protobuf field approaches; done means one supported way to match and control FK cascade plans.

Written by the indexing model from the issue text.