[Feature Request]: Design adequate yet flexible ways to serialize/deserialize DoFn payloads about beam

[sjvanrossum]

I haven't fully evaluated rkyv yet, but its rkyv_dyn crate (also uses inventory for the type registry) seems promising since it does allow registration of generic implementations of non-generic traits, e.g.:

struct MyGenericDoFn<I> {
  marker: PhantomData<I>,
}

impl<I> TypedDoFn for MyGenericDoFn<I> {
  type I = String;
  type O = String;

  fn process_element(&mut self, element: &Self::I) -> Self::O {
    element.to_uppercase()
  }
}

impl<I> UntypedDoFn for MyGenericDoFn<I> {
  fn process(&mut self, element: Box<dyn Any>) -> Box<dyn Any> {
    Box::new(self.process_element(element.downcast_ref::<<Self as DoFn>::Input>().unwrap()))
  }
}

The only requirement is that users register them with concrete types in the hopefully few cases where that'd be required. Again, perhaps some of that can also be hidden behind a macro.