Caplin: Add POST `/eth/v1/beacon/states/{state_id}/validator_balances` to the Beacon API

[Giulio2002]

Description

We need to add a new endpoint to the Beacon API that allows for fetching validator balances given a specific state ID. This will be a POST request to provide more flexibility and accommodate future payload requirements.

Endpoint

POST /eth/v1/beacon/states/{state_id}/validator_balances

Parameters

• state_id (path parameter): The identifier of the state for which the validator balances are to be fetched. This could be a state root, a block root, or a slot number.

Specification: https://ethereum.github.io/beacon-APIs/#/Beacon/postStateValidatorBalances