PoC implementation to extend the ETH watcher reward functionality to ETH/UTXO and ERC20/UTXO swaps. Additionally, this PR improves the reward protocol to allow only the takers to pay for the watcher service that they use. The problem is that if the taker wishes to use the watcher service, the watcher can either refund the taker payment (sent by the taker), or spend the maker payment (sent by the maker). During the swap, it is not known which of these payments the watcher is going to spend. Previous solution to this problem was to make both the taker and maker to include watcher rewards of equal amounts in their corresponding payments, which would later be exchanged if the swap is completed successfully, i.e. taker's reward goes to the maker and maker's reward goes to the taker. This solution was not ideal because there is little reason for the makers to include extra fees in their payments for a service that is used by the taker, even if they're going to take back the fee later. It would be better if the watcher service used by the taker is invisible to the maker. This is achieved by defining different targets for the reward while calling the payment functions of the swap contract. The target of the reward can be Contract, PaymentSender or PaymentSpender and it depends on the coin protocols of taker and maker. The updated contract can be found here.

ETH/ERC20 - ERC20/ETH

If both taker and maker uses EVM coins, the taker sends the reward (as ETH) to the Contract target with the taker payment. Later, this reward that is locked in the contract can be taken either by spending the maker payment or refunding the taker payment. The receiver of the reward is whoever spends/refunds the payments, i.e. taker takes back its own reward if it doesn't use watchers.

ETH taker / UTXO maker - ERC20 taker / UTXO maker

If the taker coin is EVM and the maker coin is Non-EVM, watcher only spends gas if it refunds the taker payment, spending the Non-EVM maker payment won't have a cost. In this case, the taker includes in the taker payment a reward (as ETH) with the target PaymentSender. This is a refund-only reward that can only be claimed with the refund function of the contract. Again, the receiver of the reward is the one who's calling the refund function, either the taker himself or a watcher. If the taker payment is spent successfully by the maker, the reward automatically goes back to the taker.

UTXO taker / ETH maker - UTXO taker / ERC20 maker

Unfortunately, for this case the maker is involved in the process which was avoided in the other two cases. The reason is that since the taker is Non-EVM, it can't send the reward (that will be required to spend the maker payment) to the contract together with the taker payment. It would be possible to add a new function to the swap contract just for this purpose, but sending a transaction just for the reward would be unnecessarily expensive. For this reason, the old solution is still used in this case: both the taker and the maker include some additional reward (in their own coins) in their respective payments. If the watcher spends the maker payment, it receives the reward either in ETH or ERC20. If the swap is completed successfully without watcher involvement, the taker will have maker's reward and the maker will have taker's reward. The reward that is sent by the maker has the target PaymentSpender and it goes to whoever spends this payment. The advantage of this approach is that it makes it possible to buy ETH/ERC20 with a Non-EVM coin without having any ETH.

Further information on enabling a watchtower node can be found here:
https://docs.google.com/document/d/1YHtsIFGkO2itPlFt72uH0oB0v_Puwtt3Cmjd6BF-fTM/edit#