Support for message retry requests

protos/SignalService.proto

@@ -12,6 +12,11 @@ message Envelope {
     PREKEY_BUNDLE       = 3;
     RECEIPT             = 5;
     UNIDENTIFIED_SENDER = 6;
+
+    // Our parser does not handle reserved in enums: DESKTOP-1569
+    // reserved           7;
+
+    PLAINTEXT_CONTENT   = 8;
   }
 
   optional Type   type            = 1;
@@ -34,6 +39,7 @@ message Content {
   optional ReceiptMessage receiptMessage               = 5;
   optional TypingMessage  typingMessage                = 6;
   optional bytes          senderKeyDistributionMessage = 7;
+  optional bytes          decryptionErrorMessage       = 8;
 }
 
 // Everything in CallingMessage must be kept in sync with RingRTC (ringrtc-node).