Handle addable field between patches

[yanganto]

Do you agree we do this first in this PR, and then, we can discuss which one will be good from Patch(add = "..."), Patch(addable_fields = "..."), and #[patch(addable)] in the next PR?

Sounds good to me.

By the way, what I meant with the patch(add = "...") was to let the dev define function to handle the add:

fn my_add_fx(a: SomeThingPatch, b: SomeThingPatch) {
  ...
}

#[derive(Patch)]
struct OtherThing {
  #[patch(add = "my_add_fx"]
  some_thing: SomeThing,
}

Originally posted by @taorepoara in #32 (comment)

Action Items

• #[patch(addable)] Add #[patch(addable)] #47
• #[patch(add=fn)] feat: 'add' field attribute for custom implementation #39