Supporting stringified json (serverlessworkflow#891)

Signed-off-by: fjtirado <ftirados@redhat.com>

Author: fjtirado

impl/model/src/main/java/io/serverlessworkflow/impl/model/jackson/JacksonModel.java

@@ -55,7 +55,7 @@ public Collection<WorkflowModel> asCollection() {
 
   @Override
   public Optional<String> asText() {
-    return node.isTextual() ? Optional.of(node.asText()) : Optional.empty();
+    return node.isTextual() ? Optional.of(node.asText()) : Optional.of(node.toString());
   }
 
   @Override

impl/model/src/main/java/io/serverlessworkflow/impl/model/jackson/JacksonModelFactory.java

@@ -15,6 +15,7 @@
  */
 package io.serverlessworkflow.impl.model.jackson;
 
+import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.node.DoubleNode;
 import com.fasterxml.jackson.databind.node.FloatNode;
@@ -86,7 +87,11 @@ public WorkflowModel from(Number number) {
 
   @Override
   public WorkflowModel from(String value) {
-    return new JacksonModel(new TextNode(value));
+    try {
+      return new JacksonModel(JsonUtils.mapper().readTree(value));
+    } catch (JsonProcessingException ex) {
+      return new JacksonModel(new TextNode(value));
+    }
   }
 
   @Override