Add binary state field to VST3 and AU plugins

[kmontag]

This adds a read-only state field and a set_state method to external plugins, similar to what's been discussed in https://github.com/spotify/pedalboard/issues/187.

This is intended as a more robust alternative to saving and loading parameter values. The value is retrieved/updated using JUCE's getStateInformation/setStateInformation.

Since it stores the plugin's entire native state (within a JUCE wrapper structure), the blob can also be used to generate preset files in other formats, e.g. vstpreset.

[kmontag]

Somehow I missed https://github.com/spotify/pedalboard/pull/289 before submitting this :)

I'll leave this PR open as it's a bit of a different approach, but feel free to close it if the other approach is preferred.

[0xdevalias]

From #289:

I have a couple high-level suggestions before I think we can merge this:

• These new methods look like they work just fine - but they require the user to save the plugin state to a file. Pedalboard's design philosophy is to integrate with Python as much as possible; in keeping with that philosophy, it might be more intuitive to change these save/load methods into a .def_property, so the plugin's state becomes a property that users can do what they like with:

  # Instead of:
  my_plugin.save_plugin_state("to/filename")
  
  # ...we could use a property:
  len(my_plugin.state) # 12,345 bytes
  with open("my_filename.sav", "wb") as f:
      f.write(my_plugin.state)
  
  # ...which also allows people to do what they want with the
  # plugin state, without requiring them to write to disk:
  my_plugin.state = some_function(my_plugin.state)  # modify the plugin's internal state

Originally posted by @psobot in https://github.com/spotify/pedalboard/issues/289#issuecomment-1959721455

---

From a quick skim of this PR, it looks like it's closer (def_property_readonly + def set_state ) to the suggested method (def_property).

I'm not sure if the 'separated read/write' approach in this PR is better than just a pure def_property.

[psobot]

Thanks @kmontag! Apologies that this took so long - I've just made a couple changes and merged this, and it should be available as of v0.9.6 (released later today).

I did rename this property to raw_state instead of just state, as the state data is often (but not always) encoded in a format that I hope we can parse and expose as a .state parameter later. (i.e.: if the state of a VST3 is valid XML, Pedalboard could unwrap and parse that XML directly to make the client code simpler.)

[psobot]

Also - I just realized that your avatar is the cover of In Absentia, one of my favourite albums; that makes me doubly sorry that this took so long. 😅

[kmontag]

Thanks all! The changes look great, glad to see this land.

@psobot 🤘 🤘