✨Introduce varGroups for configRewriter.

[calebcordry]

Vendor API

'configRewriter': {
  'url': 'https://www.amp.dev/rewriter',
  'varGroups': {
    'feature1': {
      'dr': 'DOCUMENT_REFERRER',
      'su': 'SOURCE_URL',
    }
   'feature2': {
      'id: '12345'
    }
  }

Publisher API

"configRewriter": {
  "varGroups": {
    "feature1": {
      "enabled": true
    }
    "feature2": {
      "enabled": true
    }
  }
}

Payload

"configRewriter": {
  "vars": {
    "dr": "www.example.com"
    "su": "www.foo.com"
    "id": "12345"
  }
}

Also introduces mock vendor that can be used for examples and local testing.

[lannka]

as discussed offline, it will be good to have a integration test first , so that we're confident about any new changes.

[calebcordry]

Ok this should be ready for another round of review 😀

[lannka]

@zikas FYI

[calebcordry]

@lannka this wound up being a decent size refactor (also added some more tests) PTAL.

Now we have 2 options where either pub or vendor can enable this feature. Also made sure we delete the configRewriter.vars obj if nothing has been enabled.