Expose certain validator_monitor metrics to the HTTP API (#3760)

## Issue Addressed

#3724 

## Proposed Changes

Exposes certain `validator_monitor` as an endpoint on the HTTP API. Will only return metrics for validators which are actively being monitored.

### Usage

```bash
curl -X GET "http://localhost:5052/lighthouse/ui/validator_metrics" -H "accept: application/json" | jq
```

```json
{
  "data": {
    "validators": {
      "12345": {
        "attestation_hits": 10,
        "attestation_misses": 0,
        "attestation_hit_percentage": 100,
        "attestation_head_hits": 10,
        "attestation_head_misses": 0,
        "attestation_head_hit_percentage": 100,
        "attestation_target_hits": 5,
        "attestation_target_misses": 5,
        "attestation_target_hit_percentage": 50 
      }
    }
  }
}
```

## Additional Info

Based on #3756 which should be merged first.

beacon_node/beacon_chain/src/validator_monitor.rs

@@ -629,6 +629,14 @@ impl<T: EthSpec> ValidatorMonitor<T> {
         self.validators.len()
     }
 
+    // Return the `id`'s of all monitored validators.
+    pub fn get_all_monitored_validators(&self) -> Vec<String> {
+        self.validators
+            .iter()
+            .map(|(_, val)| val.id.clone())
+            .collect()
+    }
+
     /// If `self.auto_register == true`, add the `validator_index` to `self.monitored_validators`.
     /// Otherwise, do nothing.
     pub fn auto_register_local_validator(&mut self, validator_index: u64) {