Make sure, when plugins for a GET request to fail, there is always a response body.

Also add classes to CPP API for convenient manipulation of components of HTTP messagess.
These classes may be used independently of other parts of the TS CPP API.  (This PR is
absorbing PR #5812.)

Adds Au test with plugin forcing various errors on various hooks, with default body
or set body.

Author: Walter Karas

plugins/xdebug/Cleanup.h include/tscpp/api/Cleanup.hplugins/xdebug/Cleanup.h renamed to include/tscpp/api/Cleanup.h

@@ -124,9 +124,13 @@ class TxnAuxDataMgrBase
 
 using TxnAuxMgrData = TxnAuxDataMgrBase::MgrData;
 
-// Class to manage auxiliary data for a transaction.  If an instance is created for the transaction, the instance
-// will be deleted on the TXN_CLOSE transaction hook (which is always triggered for all transactions).
-// The TxnAuxData class must have a public default constructor.
+// Class to manage auxiliary data for a transaction.  If an instance of auxiliary data is created for the
+// transaction, the instance will be deleted on the TXN_CLOSE transaction hook (which is always triggered
+// for all transactions).  The TxnAuxData class must have a public default constructor.
+//
+// This class and the base class are pseudo-namespaces, have only static member functions (no data members).
+// The manager data cannot simply be in the base class, because the (static member) continuation function that
+// runs on the TXN_CLOSE hook would not be able to access it.
 //
 template <class TxnAuxData, TxnAuxMgrData &MDRef> class TxnAuxDataMgr : private TxnAuxDataMgrBase
 {