Make `pubkey` field optional

[MegaRedHand]

Once #1067 is merged, we can make the pubkey field optional on keystores. For that, we'd need to change this fetch! here:

lambda_ethereum_consensus/lib/keystore.ex

Line 27 in 5160683

pubkey = Map.fetch!(decoded_json, "pubkey") |> parse_binary!()

[artemd24]

I'd like to tackle this!