Support local working image CDN support

[aghreed]

Purpose

Fixes an issue I introduced with #123 where locally you'll get issues running your site as the syntax will be for gatsbyImageData rather than the gatsbyImage ImageCDN query.

This also re-introduces the mode option initially purposed to control ImageCDN support and makes it so that either mode or the GATSBY_CLOUD_IMAGE_CDN env var that is provided by Gatsby Cloud during Gatsby Cloud builds will trigger the plugin to create ImageCDN compatible nodes.

Testing

Locally gatsby develop is working with a demo site I'm using, however gatsby build and gatsby serve is having issues finding the images. Putting this PR back into draft until I can get to the bottom on that.

However, on Gatsby Cloud, this is working just fine. Build URL: https://build-b55a8a8d-761a-458d-a31b-d99c233f1499.gatsbyjs.io/