Create a separate InitContext

Only a couple of these functions would be needed during initialization.
In the next couple commits ProcessContext will get a way to access
auxiliary IO, so this really had to be separated.

plugins/examples/sine/src/lib.rs

@@ -124,7 +124,7 @@ impl Plugin for Sine {
         &mut self,
         _bus_config: &BusConfig,
         buffer_config: &BufferConfig,
-        _context: &mut impl ProcessContext,
+        _context: &mut impl InitContext,
     ) -> bool {
         self.sample_rate = buffer_config.sample_rate;