Add Counterparty signing support

- Add a new parameter "signature_target" to "sign". Supports single and
  multisign, but I haven't written tests for multisign yet.
- Skip account validation if this field is set, like multisigning.
- Unit tests demonstrating examples.

include/xrpl/protocol/Protocol.h

@@ -23,6 +23,7 @@
 #include <xrpl/basics/ByteUtilities.h>
 #include <xrpl/basics/base_uint.h>
 #include <xrpl/basics/partitioned_unordered_map.h>
+#include <xrpl/protocol/Units.h>
 
 #include <cstdint>
 

include/xrpl/protocol/STObject.h

@@ -389,6 +389,8 @@ public:
     setFieldV256(SField const& field, STVector256 const& v);
     void
     setFieldArray(SField const& field, STArray const& v);
+    void
+    setFieldObject(SField const& field, STObject const& v);
 
     template <class Tag>
     void

include/xrpl/protocol/STTx.h

@@ -125,7 +125,11 @@ public:
     getJson(JsonOptions options, bool binary) const;
 
     void
-    sign(PublicKey const& publicKey, SecretKey const& secretKey);
+    sign(
+        PublicKey const& publicKey,
+        SecretKey const& secretKey,
+        std::optional<std::reference_wrapper<SField const>> signatureTarget =
+            {});
 
     enum class RequireFullyCanonicalSig : bool { no, yes };
 

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(Counterparty);         // field.
 JSS(CounterpartySignature);// field.
 JSS(DeliverMax);           // out: alias to Amount
 JSS(DeliverMin);           // in: TransactionSign
@@ -567,6 +568,7 @@ JSS(settle_delay);            // out: AccountChannels
 JSS(severity);                // in: LogLevel
 JSS(shares);                  // out: VaultInfo
 JSS(signature);               // out: NetworkOPs, ChannelAuthorize
+JSS(signature_target);        // in: TransactionSign
 JSS(signature_verified);      // out: ChannelVerify
 JSS(signing_key);             // out: NetworkOPs
 JSS(signing_keys);            // out: ValidatorList