Proto/plugin/RPC: Add an RPC to check if a delegate is forbidden

Add an RPC into the protocol plugin to check if a delegate is forbidden (using https://gitlab.com/tezos/tezos/-/blob/master/src/proto_alpha/lib_protocol/forbidden_delegates_storage.ml?ref_type=heads and https://gitlab.com/tezos/tezos/-/blob/master/src/proto_alpha/lib_protocol/alpha_context.mli?ref_type=heads#L2315)