Add gatsbyImageData resolver

[ascorbic]

Hello!
This PR adds initial support for the beta Gatsby Image plugin. This plugin includes a new, higher-performance image component that gives better Lighthouse scores, and a simpler GraphQL API. See more information here. As part of the image plugin, we have added a toolkit to help add support to source plugins. I have used this to add basic support for the new resolver. See details here: gatsbyjs/gatsby#28241

The new plugin also now has support for runtime dynamic images with a new useGatsbyImage hook. You could create a SanityImage component that takes an image node and the image options as props, rather than through GraphQL. I'd love to work with you to show how this could be done.

For the moment, while the image plugin is in beta, we should not be documenting this as the preferred method, but by the time we go to GA next month we'd like to try and start moving starters and examples over to the new plugin. We have a codemod to help users with migration, and there is a possibility to add support for migrating Sanity sites too.

I'd love some feedback on this PR, and whether you like the approach I've taken.

[coreyward]

I like the approach mapping image fit modes to Gatsby Image fit/fill styles, but I believe this will need to wait to be merged until this bug gets fixed, otherwise the fit modes will get ignored entirely.

[johngrasty]

This might be rather helpful to move forward on given that gatsby@3 is out and along with it the official blessing of gatsby-plugin-image.

[RemusDutulescu]

Also waiting for this. The newly embraced gatsby-plugin-image's GatsbyImage doesn't play well with _raw Sanity image fields...

[bjoerge]

These commits were included in today's v7.0.0 release, which also added support for Gatsby v3. (Full changelog)

Thanks a lot @ascorbic! 🙌 💖