This PR enables users to easily modify the relay.Connection type to add fields like nodes, or customize the existing Edges and PageInfo fields to make changes about nullability.
This is handy for cases like having a generic nodes field, that skips the use of edges for non-pagination use. The change is enabled by checking if _meta was already provided in the keyword arguments of Node/Connection's init_subclass_with_meta, so we don't override existing field definitions from the superclass.

The most common use case will be to add a generic nodes field for cases where cursor-based pagination is not necessary:

type MyTypeConnection {
	pageInfo: PageInfo
	edges: [MyTypeEdge]
	nodes: [MyType]
}