Log fee recipients in VC (#3526)

## Issue Addressed

Resolves #3524 

## Proposed Changes

Log fee recipient in the `Validator exists in beacon chain` log. Logging in the BN already happens here https://github.com/sigp/lighthouse/blob/18c61a5e8be3e54226a86a69b96f8f4f7fd790e4/beacon_node/beacon_chain/src/beacon_chain.rs#L3858-L3865

I also think it's good practice to encourage users to set the fee recipient in the VC rather than the BN because of issues mentioned here #3432

Some example logs from prater:
```
Aug 30 03:47:09.922 INFO Validator exists in beacon chain        fee_recipient: 0xab97_ad88, validator_index: 213615, pubkey: 0xb542b69ba14ddbaf717ca1762ece63a4804c08d38a1aadf156ae718d1545942e86763a1604f5065d4faa550b7259d651, service: duties


Aug 30 03:48:05.505 INFO Validator exists in beacon chain        fee_recipient: Fee recipient for validator not set in validator_definitions.yml or provided with the `--suggested-fee-recipient flag`, validator_index: 210710, pubkey: 0xad5d67cc7f990590c7b3fa41d593c4cf12d9ead894be2311fbb3e5c733d8c1b909e9d47af60ea3480fb6b37946c35390, service: duties
```


Co-authored-by: Paul Hauner <paul@paulhauner.com>

validator_client/src/duties_service.rs

@@ -400,13 +400,23 @@ async fn poll_validator_indices<T: SlotClock + 'static, E: EthSpec>(
                 )
                 .await;
 
+            let fee_recipient = duties_service
+                .validator_store
+                .get_fee_recipient(&pubkey)
+                .map(|fr| fr.to_string())
+                .unwrap_or_else(|| {
+                    "Fee recipient for validator not set in validator_definitions.yml \
+                    or provided with the `--suggested-fee-recipient` flag"
+                        .to_string()
+                });
             match download_result {
                 Ok(Some(response)) => {
                     info!(
                         log,
                         "Validator exists in beacon chain";
                         "pubkey" => ?pubkey,
-                        "validator_index" => response.data.index
+                        "validator_index" => response.data.index,
+                        "fee_recipient" => fee_recipient
                     );
                     duties_service
                         .validator_store
@@ -420,7 +430,8 @@ async fn poll_validator_indices<T: SlotClock + 'static, E: EthSpec>(
                     debug!(
                         log,
                         "Validator without index";
-                        "pubkey" => ?pubkey
+                        "pubkey" => ?pubkey,
+                        "fee_recipient" => fee_recipient
                     )
                 }
                 // Don't exit early on an error, keep attempting to resolve other indices.
@@ -430,6 +441,7 @@ async fn poll_validator_indices<T: SlotClock + 'static, E: EthSpec>(
                         "Failed to resolve pubkey to index";
                         "error" => %e,
                         "pubkey" => ?pubkey,
+                        "fee_recipient" => fee_recipient
                     )
                 }
             }