Support `NULLS NOT DISTINCT`

[kylepl]

Postgres 15 release 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

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