Add `schematic_attr` for forwarding attributes

[MrGVSV]

Objective

Resolves #53

Solution

Adds the schematic_attr and asset_schematic_attr attributes to the Schematic and AssetSchematic derives, respectively. This allows attributes to be forwarded to the generated input type.

For example, take the following schematic:

#[derive(Component, Schematic, Reflect)]
#[schematic_attr(
  derive(Default),
  reflect(Default),
)]
struct MyStruct {
  #[schematic(from = u8)]
  data: u32,
  #[schematic_attr(reflect(ignore))]
  hash: u64,
}

Will now generate an input type like:

#[derive(Reflect)]
#[derive(Default)]
#[reflect(Default)]
struct MyStructInput {
  data: u8,
  #[reflect(ignore)] 
  hash: u64,
}

---

This also means that reflection attributes are no longer automatically passed to the input type. That behavior was removed as it would sometimes result in invalid behavior and was also not very flexible.