feat: cube support

[jayy-lmao]

• feat: cube support

Closes https://github.com/jayy-lmao/sql-gen/issues/9 by adding postgres cubes. Note when using this you will need sqlx >= 0.8, but wont be a dep unless your db has cubes in it.

[jayy-lmao]

Wont merge until after https://github.com/jayy-lmao/sql-gen/pull/13, as https://github.com/jayy-lmao/sql-gen/issues/9 will also need enum support anyways 👍