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>