feat: support xunit from pubsub subscription

[bcoe]

This sees @cedpeters' work over the finish line, making it so we can accept data posted directly from the PubSub subscription used by our build cop bot.

The data posted from PubSub just happens to be a slightly different shape than we planned for:

{
  "message": {
     "data": "base64data"
  }
}

Otherwise I believe we can use the work as implemented.

fixes #262, #261

[bcoe]

@cedpeters a few bugs aside, I was able to wire up your XML parsing and it's working, FYI:

[cedpeters]

@bcoe That's amazing! So cool to see it in action at last!