Return the list of tokens registered with the given payment network.

def get_token_identifiers(
        chain_state: ChainState,
        payment_network_id: PaymentNetworkID,
) -> List[TokenAddress]:
    """ Return the list of tokens registered with the given payment network. """
    payment_network = chain_state.identifiers_to_paymentnetworks.get(payment_network_id)

    if payment_network is not None:
        return [
            token_address
            for token_address in payment_network.tokenaddresses_to_tokenidentifiers.keys()
        ]

    return list()