Implement Lending Protocol (unsupported) (#5270)

- Spec: XLS-66
- Introduces amendment "LendingProtocol", but leaves it UNSUPPORTED to
  allow for standalone testing, future development work, and potential
  bug fixes.
- AccountInfo RPC will indicate the type of pseudo-account when
  appropriate.
- Refactors and improves several existing classes and functional areas,
  including Number, STAmount, STObject, json_value, Asset, directory
  handling, View helper functions, and unit test helpers.

src/libxrpl/protocol/STTx.cpp

@@ -261,15 +261,12 @@ STTx::checkSign(Rules const& rules) const
     if (auto const ret = checkSign(rules, *this); !ret)
         return ret;
 
-    /* Placeholder for field that will be added by Lending Protocol
     if (isFieldPresent(sfCounterpartySignature))
     {
         auto const counterSig = getFieldObject(sfCounterpartySignature);
-        if (auto const ret = checkSign(rules, counterSig);
-            !ret)
+        if (auto const ret = checkSign(rules, counterSig); !ret)
             return Unexpected("Counterparty: " + ret.error());
     }
-    */
     return {};
 }