Add an AsyncExecutor associated type to Plugin

This will make it possible to run background tasks in a type safe way.
Sadly, this does mean that every plugin now needs to define the type
alias and constructor function since Rust does not yet support defaults
for associated types.

plugins/loudness_war_winner/src/lib.rs

@@ -123,6 +123,9 @@ impl Plugin for LoudnessWarWinner {
     const DEFAULT_INPUT_CHANNELS: u32 = 2;
     const DEFAULT_OUTPUT_CHANNELS: u32 = 2;
 
+    type AsyncExecutor = ();
+    fn async_executor(&self) -> Self::AsyncExecutor {}
+
     fn params(&self) -> Arc<dyn Params> {
         self.params.clone()
     }