Improve Invariant check handling if not in Transactor context

- Adds an option to isFeatureEnabled to return true if no global rules
  set. Can be used for functionality that uses the post-amendment
  behavior when called outside of a Transactor context.

include/xrpl/protocol/Rules.h

@@ -8,8 +8,13 @@
 
 namespace xrpl {
 
+/** Check whether a feature is enabled in the current ledger rules
+ *
+ * @param feature The feature to be tested.
+ * @param resultIfNoRules What to return if called from outside a Transactor context.
+ */
 bool
-isFeatureEnabled(uint256 const& feature);
+isFeatureEnabled(uint256 const& feature, bool resultIfNoRules = false);
 
 class DigestAwareReadView;