Helper function to get best available nodes

Add new helper function to get the best available nodes.

Use bma.network.ws2p_heads request to get the pubkeys of the most updated nodes.

Return nodes grouped by blockstamp by descending order. So the first group is the best candidate (but you can check how many nodes follow this block to be sure).

For each node, we need the endpoints do connect to. This require a bma.network.peers request.

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