Add public documented REST API to fetch data

[olisand]

As a developper of service extensions, I want to use a public well documented API, so that I can integrate my apps or widgets with the TMD.

The fetch API needs to support the following:

• Fetching questions for each available question super set
• Fetching metrics
  • Filter by question
  • Filter by node (only allowed using api token)
  • Filter by other standard filters
• The API should have CORS that allows any page to fetch the data

[pdmnyberg]

We currently have an implementation that allow a user to get the publicly available data using questions and/or supersets as filters. Currently the API requires the user to be signedin in order to get node specific results.

What is currently missing is to identify a way to allow a node admin to publish node specific data in order to make it available for a widget.