opt: include index recommendations for FK cascades

Consider the tables:

```sql
CREATE TABLE p (id INT PRIMARY KEY);

CREATE TABLE c (id INT PRIMARY KEY, p_id INT REFERENCES p(id) ON DELETE CASCADE);
```

CRDB should recommend `CREATE INDEX ON c(p_id)` to speed up the FK cascade when deleting a row from `p`. It currently makes no recommendation:

```
defaultdb> EXPLAIN DELETE FROM p WHERE id = 1;
info
--------------------------
distribution: local
vectorized: true

• root
│
├── • delete range
│ from: p
│ spans: [/1 - /1]
│
└── • fk-cascade
fk: c_p_id_fkey
(11 rows)
```

Jira issue: CRDB-27088

gz#18371