Add binary state field to VST3 and AU plugins (#297)

* Add binary state property to VST3 and AU plugins.

* Add setter for AU/VST3 state.

* Add type stubs for state and set_state

* Update docs.

* Add test.

* Rename to raw_state and add docstring.

---------

Co-authored-by: Peter Sobot <psobot@spotify.com>