Following on from: https://issues.jenkins-ci.org/browse/JENKINS-38590

When there is a step of unknown type, its json object can be rendered by the pipeline-model plugin as the Jenkinsfile form (a line of text), this can then be shown in a text box and editable (when an edit is made, it can be transformed back to the json object form by the pipeline model plugin).

In scope:

• Display text box in case of unsupported step type
• Conversion of step object to text
• Displaying of editable text
• Conversion of text to object
• Works with block scoped steps as well as simple steps

Some more details, using an example:

 "steps": [      {
        "name": "node",
        "arguments":         {
          "isConstant": true,
          "value": "some-label"
        },
        "children": [        {
          "name": "sh",
          "arguments":           {
            "isConstant": true,
            "value": "echo ONSLAVE=$ONSLAVE"
          }
        }]
      }]

(from https://github.com/jenkinsci/pipeline-model-definition-plugin/blob/master/src/test/resources/json/agentNoneWithNode.json)

In this case, it is the "node" step (say we don't have a form based editor for it). In that case the entire object that is the "node" step from that step listing can be converted to textual form and displayed in a box (and then back).

This is a good catch all for unsupported steps.