Add support for ulid data type

[duarten]

Add support for https://github.com/pksunkara/pgx_ulid. This is an extension type, so not sure how best to support it.

Alternatively, it should be possible to treat this type as a string. When I try that, however, I get:

Error { kind: ToSql(1), cause: Some(WrongType { postgres: Other(Other { name: "ulid", oid: 16389, kind: Simple, schema: "public" }), rust: "alloc::string::String" }) }