[enhancement] validator_start_index should allow taking in a sequence of int like how validator_indices behaves

[yzhuang]

The current code looks like this:
https://github.com/ethereum/staking-deposit-cli/blob/master/staking_deposit/cli/generate_bls_to_execution_change.py#L135

@click.pass_context
def generate_bls_to_execution_change(
        ctx: click.Context,
        bls_to_execution_changes_folder: str,
        chain: str,
        mnemonic: str,
        mnemonic_password: str,
        validator_start_index: int,
        validator_indices: Sequence[int],
        bls_withdrawal_credentials_list: Sequence[bytes],
        execution_address: HexAddress,
        devnet_chain_setting: str,
        **kwargs: Any):

Consider this command:

./deposit --language=english generate-bls-to-execution-change \
...
--validator_start_index=0 \
--validator_indices="50000, 50001"

validator 50000 will use start index 0, and 50001 will use start index 1. However, this assumption is not always true.

It would be nice to allow users to do:

./deposit --language=english generate-bls-to-execution-change \
...
--validator_start_index=0,3 \
--validator_indices="50000, 50009"

Or the below since some users reuse withdraw credentials for multiple deposits:

./deposit --language=english generate-bls-to-execution-change \
...
--validator_start_index=0,0 \
--validator_indices="50000, 50009"