Reserve transaction type and error codes for event hooking:

Event hooks will allow accounts to introduce configurable behavior
in response to send and receive events.

src/ripple/protocol/TxFormats.h

@@ -56,6 +56,8 @@ enum TxType {
     ttTRUST_SET = 20,
     ttACCOUNT_DELETE = 21,
 
+    ttHOOK_SET [[maybe_unused]] = 22,
+
     ttAMENDMENT = 100,
     ttFEE = 101,
     ttUNL_MODIFY = 102,