Refactor public node index fetching into separate function

[alyssadai]

I think it would make sense to handle the "getting" of the remote node index in its own function, analogous to how we handle the local nodes in parse_nodes_as_dict - and have create_federation_node_index only be concerned with stitching them together.

The node-getter-functions should probably also raise their own warnings, both to make testing easier and to keep responsibilities a bit more separated.

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