Rename PartialPendingWithdrawal field index to validator_index

[lucassaldanha]

TLDR

This PR changes PendingPartialWithdrawal.index to PendingPartialWithdrawal.validator_index. This makes it more aligned with Withdrawal and other places where we refer to ValidatorIndex as validator_index.

Justification

In Capella, we introduced Withdrawal with the following definition:

class Withdrawal(Container):
    index: WithdrawalIndex
    validator_index: ValidatorIndex
    address: ExecutionAddress
    amount: Gwei

And in Electra, we introduced PendingPartialWithdrawal as:

class PendingPartialWithdrawal(Container):
    index: ValidatorIndex
    amount: Gwei
    withdrawable_epoch: Epoch

The fact that we used Withdrawal.validator_index but PendingPartialWithdrawal.index is confusing. And can be misleading given we have Withdrawal.index with a complete different meaning.

[jtraglia]

I totally support this change. Thank you! Just a couple nits.

[ppopth]

LGTM