Add latency measurement service to VC (#4024)

## Issue Addressed

NA

## Proposed Changes

Adds a service which periodically polls (11s into each mainnet slot) the `node/version` endpoint on each BN and roughly measures the round-trip latency. The latency is exposed as a `DEBG` log and a Prometheus metric.

The `--latency-measurement-service` has been added to the VC, with the following options:

- `--latency-measurement-service true`: enable the service (default).
    - `--latency-measurement-service`: (without a value) has the same effect.
- `--latency-measurement-service false`: disable the service.

## Additional Info

Whilst looking at our staking setup, I think the BN+VC latency is contributing to late blocks. Now that we have to wait for the builders to respond it's nice to try and do everything we can to reduce that latency. Having visibility is the first step.

validator_client/src/lib.rs

@@ -8,6 +8,7 @@ mod duties_service;
 mod graffiti_file;
 mod http_metrics;
 mod key_cache;
+mod latency;
 mod notifier;
 mod preparation_service;
 mod signing_method;
@@ -563,6 +564,14 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
             None
         };
 
+        if self.config.enable_latency_measurement_service {
+            latency::start_latency_service(
+                self.context.clone(),
+                self.duties_service.slot_clock.clone(),
+                self.duties_service.beacon_nodes.clone(),
+            );
+        }
+
         Ok(())
     }
 }