Enum type support

[Tishj]

This PR adds support for Postgres's ENUM types.

In Postgres ENUM types can not be ephemeral (exist only for the duration of a query), they have to be registered, and thus we can't just recreate the ENUM type when converting back from DuckDB -> Postgres at the end of execution.

This detail caused the biggest development overhead, we need to vendor and inherit from a templated derived class that contains the guts of the DuckDB ENUM type so we can add Postgres's ENUM member oids to it.

template <class T>
class PGDuckDBEnumTypeInfo : public EnumTypeInfoTemplated<T> {
public:
	PGDuckDBEnumTypeInfo(Vector &values_insert_order_p, idx_t dict_size_p, Vector &oid_vec)
	    : EnumTypeInfoTemplated<T>(values_insert_order_p, dict_size_p), oid_vec(oid_vec) {
	}

public:
	const Vector &
	GetMemberOids() const {
		return oid_vec;
	}

private:
	Vector oid_vec;
};

We save the ENUM member oids into this, as this is what has to be returned in the query result.

[Tishj]

I'm waiting on https://github.com/duckdb/pg_duckdb/pull/97 so I can add the tests with a > 'sad' etc, that will involve adding support for looking up type catalog entries, which is probably a nice bonus (but might be better as a separate PR)

[wuputah]

Regarding the 0.2.0 milestone: while it's not a blocker for 0.1.0 but it would be great for this to land, especially considering it's basically done. Use of ENUMs is certainly not uncommon in Postgres.