Add `presentationSchema` option to workflow step

[dlongley]

A workflow step should be able to include a presentationSchema that will be run against a submitted presentation (if one is requested for that step).

The presentationSchema option should look like this:

"presentationSchema": {
  "type": "JsonSchema",
  "jsonSchema": /* JSON schema here */
}

Other schema types can be supported with different custom properties depending on the "type" used.