GraphQL v4 compat

[chillu]

Like it says on the tin :) We don't need any more excuses to ignore either GraphQL v4 or versioned-snapshots ;) /cc @unclecheese

[unclecheese]

Especially since it's looking increasingly likely that snapshots will be a requirement for any headless setup (monolithic table syncing FTW).

[mfendeksilverstripe]

Release 2.x.x (GraphQL 4 compatible)

2.x.x is meant to deliver GraphQL 4 compatible version of this module. Currently, GraphQL 4 compatible code sits on master but is in experimental state. We need to do some work to push this one over the line.

• merge 1 into master and retain any relevant changes
• review existing PR which seems to be covering the upgrade to 4
• review the code on master and make sure the implementation matches GraphQL 4 requirements (some of the code may be outdated as it was implemented before the official release of GraphQL 4)
• increase automated tests code coverage
• update documentation
• create 2 branch from master and follow up with the regular release process