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

[abonander]

An explanation of this feature should be added to the docs here:

sqlx/sqlx-core/src/from_row.rs

Lines 232 to 273 in 6fa0458

	/// #### `json`
	///
	/// If your database supports a JSON type, you can leverage `#[sqlx(json)]`
	/// to automatically integrate JSON deserialization in your [`FromRow`] implementation using [`serde`](https://docs.rs/serde/latest/serde/).
	///
	/// ```rust,ignore
	/// #[derive(serde::Deserialize)]
	/// struct Data {
	/// field1: String,
	/// field2: u64
	/// }
	///
	/// #[derive(sqlx::FromRow)]
	/// struct User {
	/// id: i32,
	/// name: String,
	/// #[sqlx(json)]
	/// metadata: Data
	/// }
	/// ```
	///
	/// Given a query like the following:
	///
	/// ```sql
	/// SELECT
	/// 1 AS id,
	/// 'Name' AS name,
	/// JSON_OBJECT('field1', 'value1', 'field2', 42) AS metadata
	/// ```
	///
	/// The `metadata` field will be deserialized used its `serde::Deserialize` implementation:
	///
	/// ```rust,ignore
	/// User {
	/// id: 1,
	/// name: "Name",
	/// metadata: Data {
	/// field1: "value1",
	/// field2: 42
	/// }
	/// }
	/// ```