bevy_reflect: Capture generic type info on opaque types (#24382)

# Objective

Resolves #24235

## Solution

This PR makes it so that opaque types (i.e. those marked with
`#[reflect(opaque)]` or implemented with `impl_reflect_opaque!`), such
as `Arc<T>`, capture generic type information when possible.

Upon reviewing the code, I don't believe this was an intentional
limitation. And instead, it was probably just forgotten about.

## Testing

I added new unit tests to ensure this works properly.