Returns 'initiator', 'mediator' or 'target' to signify the role the node has
    in a transfer. If a transfer task is not found for the secrethash then the
    function returns None

def get_transfer_role(chain_state: ChainState, secrethash: SecretHash) -> Optional[str]:
    """
    Returns 'initiator', 'mediator' or 'target' to signify the role the node has
    in a transfer. If a transfer task is not found for the secrethash then the
    function returns None
    """
    task = chain_state.payment_mapping.secrethashes_to_task.get(secrethash)
    if not task:
        return None
    return role_from_transfer_task(task)