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/gain_gui_iced/src/lib.rs

@@ -90,7 +90,7 @@ impl Plugin for Gain {
         &mut self,
         _bus_config: &BusConfig,
         buffer_config: &BufferConfig,
-        _context: &mut impl ProcessContext,
+        _context: &mut impl InitContext,
     ) -> bool {
         // TODO: How do you tie this exponential decay to an actual time span?
         self.peak_meter_decay_weight = 0.9992f32.powf(44_100.0 / buffer_config.sample_rate);