Builds a POOL_RESTART request

    :param submitter_did: Id of Identity that sender transaction
    :param action       : Action that pool has to do after received transaction.
                          Can be "start" or "cancel"
    :param datetime           : Time when pool must be restarted.

async def build_pool_restart_request(submitter_did: str, action: str, datetime: str) -> str:
    """
    Builds a POOL_RESTART request

    :param submitter_did: Id of Identity that sender transaction
    :param action       : Action that pool has to do after received transaction.
                          Can be "start" or "cancel"
    :param datetime           : Time when pool must be restarted.
    """

    logger = logging.getLogger(__name__)
    logger.debug("build_pool_restart_request: >>> submitter_did: %r, action: %r, datetime: %r")

    if not hasattr(build_pool_restart_request, "cb"):
        logger.debug("build_pool_restart_request: Creating callback")
        build_pool_restart_request.cb = create_cb(CFUNCTYPE(None, c_int32, c_int32, c_char_p))

    c_submitter_did = c_char_p(submitter_did.encode('utf-8'))
    c_action = c_char_p(action.encode('utf-8'))
    c_datetime = c_char_p(datetime.encode('utf-8')) if datetime else None

    request_json = await do_call('indy_build_pool_restart_request',
                                 c_submitter_did,
                                 c_action,
                                 c_datetime,
                                 build_pool_restart_request.cb)

    res = request_json.decode()
    logger.debug("build_pool_upgrade_request: <<< res: %r", res)
    return res