refactor: Add support for extra transaction signature validation (#5851)

- Restructures `STTx` signature checking code to be able to handle
  a `sigObject`, which may be the full transaction, or may be an object
  field containing a separate signature. Either way, the `sigObject` can
  be a single- or multi-sign signature.
- This is distinct from 550f90a75e (#5594), which changed the check in
  Transactor, which validates whether a given account is allowed to sign
  for the given transaction. This cryptographically checks the signature
  validity.

src/libxrpl/protocol/STObject.cpp

@@ -688,6 +688,16 @@ STObject::getFieldV256(SField const& field) const
     return getFieldByConstRef<STVector256>(field, empty);
 }
 
+STObject
+STObject::getFieldObject(SField const& field) const
+{
+    STObject const empty{field};
+    auto ret = getFieldByConstRef<STObject>(field, empty);
+    if (ret != empty)
+        ret.applyTemplateFromSField(field);
+    return ret;
+}
+
 STArray const&
 STObject::getFieldArray(SField const& field) const
 {
@@ -833,6 +843,12 @@ STObject::setFieldArray(SField const& field, STArray const& v)
     setFieldUsingAssignment(field, v);
 }
 
+void
+STObject::setFieldObject(SField const& field, STObject const& v)
+{
+    setFieldUsingAssignment(field, v);
+}
+
 Json::Value
 STObject::getJson(JsonOptions options) const
 {