Method a calls Contract B's method b, forwarding the volta address.
Call chain: Volta -> A.a -> B.b(volta, value)
B.b requires volta.require_auth(), so when invoking through Volta,
proper auth_entries must be provided to authorize the sub-call.
fn a(
env: soroban_sdk::Env,
contract_b: soroban_sdk::Address,
volta: soroban_sdk::Address,
value: u64,
) -> u64