Fetching nested connections to nodes that can't be fetched in a flat list

[TylerBarnes]

Fetching nested connections to nodes that can't be fetched in a flat list causes nodes to be missed on the Gatsby side.

For example, in previous versions of WPGraphQL, nested menu items on menus couldn't be fetched because there was no way to properly fetch those nodes in a flat list.

What we could do to account for this situation is: as nodes are sourced and analyzed, store up every connected id fetched on every node. Once node sourcing is complete, check to see if we fetched all those nodes. For the nodes that weren't fetched, try to fetch them. If any of them come back null, mark that they're unfetchable, for the ones that do return data, create nodes. Then repeat on any missing child node ids until we have nodes for all child id's.

This is actually not a problem in WPGQL core anymore since everything can be fetched in a flat list now. I'm sure this will be needed to support WPGQL extensions though.