Decouple key fetcher; Update ECMG API

protos/public/media_cas.proto

@@ -0,0 +1,21 @@
+////////////////////////////////////////////////////////////////////////////////
+// Copyright 2018 Google LLC.
+//
+// This software is licensed under the terms defined in the Widevine Master
+// License Agreement. For a copy of this agreement, please contact
+// widevine-licensing@google.com.
+////////////////////////////////////////////////////////////////////////////////
+
+
+syntax = "proto2";
+
+package widevine.cas;
+
+// Widevine private data in the CA descriptor.
+message CaDescriptorPrivateData {
+  // Provider name.
+  optional string provider = 1;
+
+  // Content ID.
+  optional bytes content_id = 2;
+}