EXCLUDE indexes support for PostgreSQL dialect

[dezhin]

Thanks for adding INCLUDE indexes in #2855, we've updated and now it works. There are EXCLUDE indexes in PostgreSQL, which would be great to support. Here is the example:

Python 3.8.10 (default, Nov 22 2023, 10:22:35) 
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sqlglot
>>> sqlglot.parse_one("""
... CREATE TABLE ht (
...         vid INTEGER NOT NULL, 
...         fid INTEGER NOT NULL, 
...         nid INTEGER NOT NULL CHECK (nid > vid), 
...         vop CHAR(1) NOT NULL, 
...         nop CHAR(1) NOT NULL, 
...         geom GEOMETRY(POINTZ, 3857), 
...         fld_i INTEGER, 
...         fld_t TEXT, 
...         fld_d DATE, 
...         PRIMARY KEY (vid, fid), 
...         CONSTRAINT ht_vid_nid_fid_idx EXCLUDE USING gist (int4range(vid, nid) WITH &&, int4range(fid, fid, '[]') WITH &&)
... )
... """, read="postgres")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/ngw/env/lib/python3.8/site-packages/sqlglot/__init__.py", line 124, in parse_one
    result = dialect.parse(sql, **opts)
  File "/opt/ngw/env/lib/python3.8/site-packages/sqlglot/dialects/dialect.py", line 477, in parse
    return self.parser(**opts).parse(self.tokenize(sql), sql)
  File "/opt/ngw/env/lib/python3.8/site-packages/sqlglot/parser.py", line 1099, in parse
    return self._parse(
  File "/opt/ngw/env/lib/python3.8/site-packages/sqlglot/parser.py", line 1165, in _parse
    expressions.append(parse_method(self))
  File "/opt/ngw/env/lib/python3.8/site-packages/sqlglot/parser.py", line 1367, in _parse_statement
    return self.STATEMENT_PARSERS[self._prev.token_type](self)
  File "/opt/ngw/env/lib/python3.8/site-packages/sqlglot/parser.py", line 616, in <lambda>
    TokenType.CREATE: lambda self: self._parse_create(),
  File "/opt/ngw/env/lib/python3.8/site-packages/sqlglot/parser.py", line 1487, in _parse_create
    this = self._parse_schema(this=table_parts)
  File "/opt/ngw/env/lib/python3.8/site-packages/sqlglot/parser.py", line 4288, in _parse_schema
    args = self._parse_csv(lambda: self._parse_constraint() or self._parse_field_def())
  File "/opt/ngw/env/lib/python3.8/site-packages/sqlglot/parser.py", line 5367, in _parse_csv
    parse_result = parse_method()
  File "/opt/ngw/env/lib/python3.8/site-packages/sqlglot/parser.py", line 4288, in <lambda>
    args = self._parse_csv(lambda: self._parse_constraint() or self._parse_field_def())
  File "/opt/ngw/env/lib/python3.8/site-packages/sqlglot/parser.py", line 4453, in _parse_constraint
    return self.expression(
  File "/opt/ngw/env/lib/python3.8/site-packages/sqlglot/parser.py", line 1229, in expression
    return self.validate_expression(instance)
  File "/opt/ngw/env/lib/python3.8/site-packages/sqlglot/parser.py", line 1249, in validate_expression
    self.raise_error(error_message)
  File "/opt/ngw/env/lib/python3.8/site-packages/sqlglot/parser.py", line 1209, in raise_error
    raise error
sqlglot.errors.ParseError: Required keyword: 'expressions' missing for <class 'sqlglot.expressions.Constraint'>. Line 13, Col: 38.
   TEXT, 
        fld_d DATE, 
        PRIMARY KEY (vid, fid), 
        CONSTRAINT ht_vid_nid_fid_idx EXCLUDE USING gist (int4range(vid, nid) WITH &&, int4range(fid, fid, '[]') WITH &&)
)

>>> sqlglot.__version__
'22.2.1'