Make LoanSet.CounterpartySignature optional in the Tx layout

- Still required for the transaction to succeed (except inside a Batch,
  because the batch signers take care of that).
- Started adding tests for Loan-related RPC and low-level math checks.
  Currently only implemented "sign" on a LoanSet to verify it can be
  done.

include/xrpl/protocol/detail/transactions.macro

@@ -904,7 +904,7 @@ TRANSACTION(ttLOAN_SET, 80, LoanSet,
     {sfLoanBrokerID, soeREQUIRED},
     {sfData, soeOPTIONAL},
     {sfCounterparty, soeOPTIONAL},
-    {sfCounterpartySignature, soeREQUIRED},
+    {sfCounterpartySignature, soeOPTIONAL},
     {sfLoanOriginationFee, soeOPTIONAL},
     {sfLoanServiceFee, soeOPTIONAL},
     {sfLatePaymentFee, soeOPTIONAL},

include/xrpl/protocol/jss.h

@@ -59,6 +59,7 @@ JSS(BaseAsset);            // in: Oracle
 JSS(BidMax);               // in: AMM Bid
 JSS(BidMin);               // in: AMM Bid
 JSS(ClearFlag);            // field.
+JSS(CounterpartySignature);// field.
 JSS(DeliverMax);           // out: alias to Amount
 JSS(DeliverMin);           // in: TransactionSign
 JSS(Destination);          // in: TransactionSign; field.