Ask to join (#4)

This adds a new callback `joinRequest` and a new API function `Discord_Respond` to reply to it.

src/serialization.cpp

@@ -197,3 +197,32 @@ size_t JsonWriteSubscribeCommand(char* dest, size_t maxLen, int nonce, const cha
 
     return writer.Size();
 }
+
+size_t JsonWriteJoinReply(char* dest, size_t maxLen, const char* userId, int reply, int nonce)
+{
+    JsonWriter writer(dest, maxLen);
+
+    {
+        WriteObject obj(writer);
+
+        WriteKey(writer, "cmd");
+        if (reply == DISCORD_REPLY_YES) {
+            writer.String("SEND_ACTIVITY_JOIN_INVITE");
+        }
+        else {
+            writer.String("CLOSE_ACTIVITY_JOIN_REQUEST");
+        }
+
+        WriteKey(writer, "args");
+        {
+            WriteObject args(writer);
+
+            WriteKey(writer, "user_id");
+            writer.String(userId);
+        }
+
+        JsonWriteNonce(writer, nonce);
+    }
+
+    return writer.Size();
+}