Add NotaryAssisted transaction attribute type (#983)

Port neo-project/neo#3175.
Ref. neo-project/neo#2896.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>
Co-authored-by: Shargon <shargon@gmail.com>

src/Neo.SmartContract.Framework/Native/TransactionAttributeType.cs

@@ -34,6 +34,11 @@ public enum TransactionAttributeType : byte
         /// <summary>
         /// Indicates that the transaction conflicts with <see cref="Conflicts.Hash"/>.
         /// </summary>
-        Conflicts = 0x21
+        Conflicts = 0x21,
+
+        /// <summary>
+        /// Indicates that the transaction is aimed to service notary request with <see cref="NotaryAssisted.NKeys" number of keys/>.
+        /// </summary>
+        NotaryAssisted = 0x22
     }
 }