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/examples/gain/src/lib.rs

@@ -132,6 +132,12 @@ impl Plugin for Gain {
     // splits.
     const SAMPLE_ACCURATE_AUTOMATION: bool = true;
 
+    // More advanced plugins can use this to run expensive background tasks. See the `AsyncExecutor`
+    // trait's documentation for more information. `()` means that the plugin does not have any
+    // background tasks.
+    type AsyncExecutor = ();
+    fn async_executor(&self) -> Self::AsyncExecutor {}
+
     fn params(&self) -> Arc<dyn Params> {
         self.params.clone()
     }