Add Counterparty signing support

- Add a new parameter "signature_target" to "sign". Supports single and
  multisign, but I haven't written tests for multisign yet.
- Skip account validation if this field is set, like multisigning.
- Unit tests demonstrating examples.

include/xrpl/protocol/STObject.h

@@ -389,6 +389,8 @@ public:
     setFieldV256(SField const& field, STVector256 const& v);
     void
     setFieldArray(SField const& field, STArray const& v);
+    void
+    setFieldObject(SField const& field, STObject const& v);
 
     template <class Tag>
     void