Add "definitions" field to Ditto features pointing to Vorto functionblocks

[thjaeckle]

First step towards Eclipse Vorto integration:

• add an optional field "definitions" in features aside of "properties" containing a list of string identifiers targeting Vorto Functionblocks

Format:

"features": {
  "my-lamp": {
    "definition": [ "org.eclipse.example:Lamp:1.0.0" ],
    "properties": {
      ....
    }
  }
}