Add new JSONContext step validating JSON node against schema

In order to ease sub payload and metadata payload validation.

```gherkin
the JSON node "Item[0]->SubItem" should be valid according to the schema "relative_path.schema.json"
```

And its reciprocity
```gherkin
the JSON node "Item[0]->SubItem" should not be valid according to the schema "relative_path.schema.json"
```

You could add inspector validation into a private method to avoid repeating theses part of code.