Add with attribute for field transformations in valuable derive macro

[lukas-reineke]

Introduces a new #[valuable(with = "function")] attribute that allows custom field transformations during valuable serialization. Similar to serdes #[serde(serialize_with = "function")].
Functions take a reference to the field and return any type implementing Valuable.

Example

  fn debug_format<T: std::fmt::Debug>(value: &T) -> String {
      format!("{value:?}")
  }

  #[derive(Valuable)]
  struct Foo {
      #[valuable(with = "debug_format")]
      bar: HashMap<String, bool>,
  }

As far as I can tell, this works, but this is the first time I'm writing a proc macro, it's very likely that I'm missing something.