Add support for resolving ProviderConfig references

[negz]

Edit: #40 removed support for resolving ProviderConfig references per the details below. I'm repurposing this issue to track adding them back once crossplane/crossplane#2255 is solved.

What problem are you facing?

Per crossplane/crossplane#2255 it's challenging for xgql to resolve a managed resource's spec.providerConfigRef because we need to do a bunch of spelunking to figure out which provider a managed resource belongs to.

How could xgql help solve your problem?

I think the right way to solve this is to have Crossplane give us the information we need. In the meantime, we should consider removing support for resolving a managed resource's provider config from the schema. We could still expose the information as an unresolved reference in the meantime, though doing so would be a little unidiomatic for xgql.