ALTER TYPE DROP VALUE does not support "IF EXISTS" suffix

**Is your feature request related to a problem? Please describe.**

It's extremely useful for DDL statements to be idempotent - the `IF NOT EXISTS` syntax is a part of CockroachDB and Postgres as a whole that makes this feasible.

**Describe the solution you'd like**

Postgres' interface has this syntax already, but:

https://www.postgresql.org/docs/current/sql-altertype.html

```
ALTER TYPE name DROP ATTRIBUTE [ IF EXISTS ] attribute_name [ CASCADE | RESTRICT ]
```

By supporting this interface, it would be much easier to issue "idempotent DDL statements" to CockroachDB.

Jira issue: CRDB-36872

Epic CRDB-60817