Add json(nullable) macro attribute

[seanaye]

Does your PR solve an issue?

fixes #2849

This MR introduces a #[sqlx(json(nullable))] attribute which allows for the correct behaviour on

#[derive(Serialize, Deserialize)]
enum Foo {
  Bar,
  Baz
}

struct Test {
  #[sqlx(json(nullable))]
  hello: Option<Foo>
}

by allowing the underlying row value to be null