add unified slow-block log

would be good for us to participate in https://ethresear.ch/t/a-small-step-towards-data-driven-protocol-decisions-unified-slowblock-metrics-across-clients/23907

and add the slow-block log (all other EL clients have PRs for it)

as per a recent conversation https://discord.com/channels/1359927674746835211/1468942379762319423/1469305135761985633 this will allow us to leverage "Block Logs Analysis" in Benchmarkoor (e.g. look for that section in this run for Geth https://tries-screens-notify-efficiency.trycloudflare.com/runs/1770366905_a3b3d7bb_geth)