Expose withdrawal creds list validators api

[drohit-cb]

What changed? Why?

This PR helps expose withdrawal credentials of ethereum validators in preparation of Pectra. Using this customers can tell if their validator is a 0x01 or a 0x02 validator.

testing

Example local run

> import { Coinbase, ExternalAddress, StakingReward, StakeOptionsMode, Validator, ValidatorStatus, Transaction, Asset, Wallet } from "./src";
undefined

> const coinbase = Coinbase.configureFromJson({ filePath: "~/code/cb/staking/.coinbase_cloud_api_key_production.json" });
undefined

> let validators = await Validator.list("ethereum-mainnet", "eth", ValidatorStatus.ACTIVE);
undefined

> validators.forEach(validator => {console.log(validator.getWithdrawalCredentials()  + " " + validator.getWithdrawalCredentials());});
0x0100000000000000000000007f639032a6d973044b27f90a522b5b4d81466906 0x0100000000000000000000007f639032a6d973044b27f90a522b5b4d81466906

Qualified Impact

[cb-heimdall]

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