Use latency service from LH VC

Lighthouse's VC has a latency service that logs latency and adds it to the metrics for observability: https://github.com/sigp/lighthouse/blob/ceb1706c9e732d32548cbf53e201e4aecb9813dc/validator_client/src/latency.rs#L15

We want to use this as well, but currently, we'd need to copy the code, so we should find a new home for the LH code so that we can reuse it.