Empty list [] to return all validators balances (#7474)

The endpoint `/eth/v1/beacon/states/head/validator_balances` returns an empty data when the data field is `[]`. According to the beacon API spec, it should return the balances of all validators:

Reference: https://ethereum.github.io/beacon-APIs/#/Beacon/postStateValidatorBalances
`If the supplied list is empty (i.e. the body is []) or no body is supplied then balances will be returned for all validators.`


  This PR changes so that: `curl -X 'POST' 'http://localhost:5052/eth/v1/beacon/states/head/validator_balances' -d '[]' | jq` returns balances of all validators.

common/eth2/src/types.rs

@@ -688,7 +688,7 @@ pub struct ValidatorBalancesQuery {
     pub id: Option<Vec<ValidatorId>>,
 }
 
-#[derive(Clone, Serialize, Deserialize)]
+#[derive(Clone, Default, Serialize, Deserialize)]
 #[serde(transparent)]
 pub struct ValidatorBalancesRequestBody {
     pub ids: Vec<ValidatorId>,