js: Add convenient construction of rawPayload messages (#1539)

Part of svix/monorepo-private#9470.

javascript/src/index.ts

@@ -699,6 +699,31 @@ class Message {
   }
 }
 
+/**
+ * Creates a `MessageIn` with the payload already being serialized.
+ *
+ * The payload is not normalized on the server (usually whitespace outside
+ * of string literals, unnecessarily escaped characters in string and such
+ * are fixed up by the server), and is not even required to be JSON.
+ *
+ * @param payload Serialized message payload
+ * @param contentType Content type of the payload to send as a header. Defaults to `application/json`.
+ *
+ * See the class documentation for details about the other parameters.
+ */
+export function messageInRaw(eventType: string, payload: string, contentType?: string): MessageIn {
+  const headers = contentType ? { "content-type": contentType } : undefined;
+
+  return {
+    eventType,
+    payload: {},
+    transformationsParams: {
+      rawPayload: payload,
+      headers,
+    }
+  };
+}
+
 class MessageAttempt {
   private readonly api: MessageAttemptApi;