feat: add .store attribute to retrieve store input

[kylebarron]

Change list

• Store PyAsyncStorage object within PyAsyncArray and PyAsyncGroup. Store PySyncStorage object within PyArray and PyGroup. This is needed because otherwise the store input is type erased. It's fully converted to a Rust type-erased Arc which doesn't know which input Python type it would need to be associated back with. Instead, we can return the user's input object directly.
• Add .store getters to retrive input store
• Add conversions from PyAsyncStorage and PySyncStorage back to Python objects.

This will be needed for pickling anyways, see #89

Closes #86