Implement Sapling note decryption using full viewing key.

src/zcash/Note.hpp

@@ -133,6 +133,13 @@ public:
         const uint256 &epk
     );
 
+    static boost::optional<SaplingNotePlaintext> decryptUsingFullViewingKey(
+        const SaplingEncCiphertext &ciphertext,
+        const uint256 &epk,
+        const uint256 &esk,
+        const uint256 &pk_d
+    );
+
     boost::optional<SaplingNote> note(const SaplingIncomingViewingKey& ivk) const;
 
     virtual ~SaplingNotePlaintext() {}