add new phase to WasmPlugin (#3143)

* add new phase  to WasmPlugin

* fix build

* rename to INITIAL

Author: zirain

extensions/v1alpha1/wasm.pb.go

@@ -414,6 +414,9 @@ enum PluginPhase {
 
   // Insert plugin before Istio stats filters and after Istio authorization filters.
   STATS = 3;
+
+  // Insert plugin at the start of the filter chain.
+  INITIAL = 4;
 }
 
 // The pull behaviour to be applied when fetching a Wam module,