Add ability to get dataurl for images/assets via GraphQL

[andrewfairlie]

Many placeholder image tools (such as Next.js 11's blur placeholder) require the placeholder to be a Base64 data URL. With Twig you can use dataurl(URLHERE) to get that, and it would be great if this feature could be exposed via GraphQL also.

Ideally, we'd be able to do something like this...

    url
    dataurl @transform(width: 8)

If this already exists, and I've been blind, sorry!

[andris-sevcenko]

There's a discussion around this already: #9116