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/InnerObjectFormats.cpp

@@ -153,6 +153,14 @@ InnerObjectFormats::InnerObjectFormats()
             {sfBookDirectory, soeREQUIRED},
             {sfBookNode, soeREQUIRED},
         });
+
+    add(sfCounterpartySignature.jsonName,
+        sfCounterpartySignature.getCode(),
+        {
+            {sfSigningPubKey, soeOPTIONAL},
+            {sfTxnSignature, soeOPTIONAL},
+            {sfSigners, soeOPTIONAL},
+        });
 }
 
 InnerObjectFormats const&