Allow GetValidatorByIndex in the beaconapiclient package to accept multiple validator indexes

[faheelsattar]

Currently GetValidatorByIndex in the beaconapiclient package only allows to query a single validator. Since the ethereum beacon api spec allows for retrieval of more then 1 validator in 1 request, we can optimize it and bulk query. We are also using beaconapiclient in Observer and this optimization will greatly help it there as well.