[Caplin] Add support to query balances/effective balances over a range of epochs/slots

# Rationale
We regularly require the timeseries of all balances for given validator(s) over a range of epochs and slots, so we can analyse the rewards earned by our validators. Ideally behaviour would be similar to fashion as the pre-existing ```/eth/v1/beacon/states/{state_id}/validator_balances``` API call, with the exclusion of state_id argument