fix: add sorbet-schema shim for better T:Struct extension compatability

[maxveldink]

Closes #87

There has been issues with the deserialize_from method on T::Struct in downstream projects where the type had to be cast for the struct. We can use T.attached_class for this, however it complicates the type check for the underlying serializers here.

Further, we have an issue of the sigs for the T::Struct extension not being picked up by Tapioca and a shim needing to be in place for that.

Instead of adding sigs to the extension, I'm going to try and export the rbi as a shim that should be picked up by Tapioca, and use a slightly less safe call in order to give better type information to callers.