Add task reference to signatures [part 1/2] (#392)

* Reimplements property on SignRequest to hold TaskId/GeneratedFromTask

src/Storage.Interface/Models/SignRequest.cs

@@ -15,6 +15,12 @@ public class SignRequest
         [JsonProperty(PropertyName = "signatureDocumentDataType")]
         public string SignatureDocumentDataType { get; set; }
 
+        /// <summary>
+        /// The task which should be linked to this signature
+        /// </summary>
+        [JsonProperty(PropertyName = "generatedFromTask")]
+        public string GeneratedFromTask { get; set; } = string.Empty;
+
         /// <summary>
         /// List of dataElementSignatures
         /// </summary>