Proposer preparation data quoted validator index in API (#3080)

## Issue Addressed

#3077

## Proposed Changes

Quotes around validator index in `prepare_beacon_proposer` endpoint


Co-authored-by: realbigsean <sean@sigmaprime.io>

consensus/types/src/proposer_preparation_data.rs

@@ -6,6 +6,7 @@ use serde::{Deserialize, Serialize};
 #[derive(PartialEq, Debug, Serialize, Deserialize, Clone)]
 pub struct ProposerPreparationData {
     /// The validators index.
+    #[serde(with = "eth2_serde_utils::quoted_u64")]
     pub validator_index: u64,
     /// The fee-recipient address.
     pub fee_recipient: Address,