plpgsql: implement cursor declaration with arguments

Postgres allows declaring a cursor with placeholders, which are filled in by arguments supplied to the OPEN statement. This issue tracks adding support for this feature. Postgres docs here: https://www.postgresql.org/docs/current/plpgsql-cursors.html#PLPGSQL-CURSOR-DECLARATIONS.

Ex:
```
DECLARE
foo CURSOR (x INT, y INT) FOR SELECT x + y;
BEGIN
OPEN foo (1, 2);
END
```

Jira issue: CRDB-35331