Support `NULLS NOT DISTINCT`

[Postgres 15 release](https://www.postgresql.org/docs/15/release-15.html) added support for `NULLS NOT DISTINCT`.

For instance:
```
CREATE TABLE IF NOT EXISTS usage_tracking4 (
a INT PRIMARY KEY,
b INT,

UNIQUE NULLS NOT DISTINCT (a, b)
);
```

this currently fails because of a syntax error, but works fine on Postgres proper.

I tried to see if there was an on-going FR for this, but did not find it.

Jira issue: CRDB-34239

Epic CRDB-60817

Use the PostgreSQL 15 release note and the supplied CREATE TABLE example as the behavioral reference. Locate CockroachDB's CREATE TABLE and UNIQUE-constraint syntax handling, add coverage for UNIQUE NULLS NOT DISTINCT, and verify the example no longer raises a syntax error.

Written by the indexing model from the issue text.