fix(postgres)!: Preserve quoting for user defined types

[VaggelisD]

Fixes #5212

I went ahead and fixed this for Postgres only; It seems to be one of the few dialects that supports user defined types and the identifier resolution is respected:

postgres> CREATE TYPE "fOo";
CREATE TYPE

postgres> SELECT CAST(1 AS foo);
ERROR:  type "foo" does not exist
LINE 1: SELECT CAST(1 AS foo);
                         ^
postgres> SELECT CAST(1 AS "fOo");
ERROR:  type "fOo" is only a shell
LINE 1: SELECT CAST(1 AS "fOo");

DuckDB also supports UDTs but quoting doesn't seem to make a difference:

duckdb> CREATE TYPE "fOo" AS INTEGER;

duckdb>  SELECT CAST (1 AS foo);
┌────────────────┐
│ CAST(1 AS foo) │
│     int32      │
├────────────────┤
│       1        │
└────────────────┘

duckdb>  SELECT CAST (1 AS "Foo");
┌────────────────┐
│ CAST(1 AS Foo) │
│     int32      │
├────────────────┤
│       1        │
└────────────────┘

Lastly, T-SQL also allows UDTs, but the approach of this PR wouldn't work because T-SQL also supports such statements (notice the builtin quoted type)

CREATE TABLE [mytable]([email] [varchar](255) NOT NULL)

Which would be transpiled as the identifier "VARCHAR" to other dialects.

[georgesittas]

DuckDB also supports UDTs but quoting doesn't seem to make a difference: [...]

What if you include unsafe characters though that need to be delimited? E.g. "foo bar".

[georgesittas]

@VaggelisD this fails in duckdb now:

>>> import sqlglot
>>> sqlglot.parse_one('cast(x as "foo bar")', 'duckdb')
Cast(
  this=Column(
    this=Identifier(this=x, quoted=False)),
  to=DataType(this=Type.USERDEFINED, kind=foo bar))
>>> sqlglot.parse_one('cast(x as "foo bar")', 'duckdb').sql('duckdb')
'CAST(x AS foo bar)'