Implement support for `Box<[T]>` and `Arc<[T]>`

Arc<[T]> is especially helpful in cases where something needs to be sent into a different thread, but cloning and double indirection are undesirable.

Right now both fail something like this:

error[E0277]: the trait bound `[subspace_core_primitives::pieces::PieceIndex]: parity_scale_codec::WrapperTypeDecode` is not satisfied
  --> crates/subspace-networking/src/protocols/request_response/handlers/cached_piece_by_index.rs:28:24
   |
28 |     pub cached_pieces: Box<[PieceIndex]>,
   |                        ^^^^^^^^^^^^^^^^^ the trait `parity_scale_codec::WrapperTypeDecode` is not implemented for `[subspace_core_primitives::pieces::PieceIndex]`, which is required by `std::boxed::Box<[subspace_core_primitives::pieces::PieceIndex]>: parity_scale_codec::Decode`
   |
   = help: the following other types implement trait `parity_scale_codec::WrapperTypeDecode`:
             std::boxed::Box<T>
             std::rc::Rc<T>
             std::sync::Arc<T>
   = note: required for `[subspace_core_primitives::pieces::PieceIndex]` to implement `parity_scale_codec::Decode`
   = note: 1 redundant requirement hidden
   = note: required for `std::boxed::Box<[subspace_core_primitives::pieces::PieceIndex]>` to implement `parity_scale_codec::Decode`

I'm not 100% sure, but suspect it is an upstream issue since downstream users will not be able to implement WrapperTypeDecode for Box<[T]> or Arc<[T]>.

Start with the parity_scale_codec::WrapperTypeDecode trait and the Box/Arc decoding behavior implicated by the error in crates/subspace-networking/src/protocols/request_response/handlers/cached_piece_by_index.rs. Reproduce that example, then verify that Box and Arc support the requested decoding cases and that the existing codec checks still pass.