Tweaks to reward APIs (#3957)

## Proposed Changes

* Return the effective balance in gwei to align with the spec ([ideal attestation rewards](https://ethereum.github.io/beacon-APIs/?urls.primaryName=dev#/Rewards/getAttestationsRewards)).
* Use quoted `i64`s for attestation and sync committee rewards.

common/eth2/src/lighthouse/attestation_rewards.rs

@@ -28,8 +28,10 @@ pub struct TotalAttestationRewards {
     #[serde(with = "eth2_serde_utils::quoted_u64")]
     pub head: u64,
     // attester's reward for target vote in gwei
+    #[serde(with = "eth2_serde_utils::quoted_i64")]
     pub target: i64,
     // attester's reward for source vote in gwei
+    #[serde(with = "eth2_serde_utils::quoted_i64")]
     pub source: i64,
     // TBD attester's inclusion_delay reward in gwei (phase0 only)
     // pub inclusion_delay: u64,

common/eth2/src/lighthouse/sync_committee_rewards.rs

@@ -8,5 +8,6 @@ pub struct SyncCommitteeReward {
     #[serde(with = "eth2_serde_utils::quoted_u64")]
     pub validator_index: u64,
     // sync committee reward in gwei for the validator
+    #[serde(with = "eth2_serde_utils::quoted_i64")]
     pub reward: i64,
 }