Cockroach: opt: support index hints on DELETE/UPDATE

[justinj]

Today, we silently ignore index hints on UPDATE/DELETE:

CREATE TABLE x (
  a INT PRIMARY KEY,
  b INT,
  c INT,
  INDEX b_idx (b),
  INDEX c_idx (c)
)
----

opt
DELETE FROM
    x@primary
WHERE
    a = 1 AND b = 2 AND c = 3
----
delete x
 ├── columns: <none>
 ├── fetch columns: a:4(int) b:5(int) c:6(int)
 ├── cardinality: [0 - 0]
 ├── side-effects, mutations
 └── select
      ├── columns: a:4(int!null) b:5(int!null) c:6(int!null)
      ├── cardinality: [0 - 1]
      ├── key: ()
      ├── fd: ()-->(4-6)
      ├── index-join x
      │    ├── columns: a:4(int!null) b:5(int) c:6(int)
      │    ├── cardinality: [0 - 1]
      │    ├── key: ()
      │    ├── fd: ()-->(4-6)
      │    └── scan x@b_idx
      │         ├── columns: a:4(int!null) b:5(int!null)
      │         ├── constraint: /5/4: [/2/1 - /2/1]
      │         ├── cardinality: [0 - 1]
      │         ├── key: ()
      │         └── fd: ()-->(4,5)
      └── filters
           └── c = 3 [type=bool, outer=(6), constraints=(/6: [/3 - /3]; tight), fd=()-->(6)]

We should use the index requested by the hint or explicitly error out when one is provided (preferably the former)

[awoods187]

We should extend index hints to work here.