Adding RewardsWithdrawn event to TACoApp contract

[manumonti]

Interface for Threshold applications includes an event RewardsWithdrawn intended to be emitted with the withdrawRewards is called (if it succeeds).

https://github.com/threshold-network/solidity-contracts/blob/8fcf42c6fcb50bb7730155bd5b99239b7b30382d/contracts/staking/IApplication.sol#L26

But in TACo application this event is not emitted. RewardPaid is emitted instead.

nucypher-contracts/contracts/contracts/TACoApplication.sol

Line 559 in 07ce241

emit RewardPaid(msg.sender, _stakingProvider, beneficiary, value);

So, if we want to fit with this interface, a RewardsWithdrawn event should be emitted.

Considering that IApplication's RewardsWithdrawn has two parameters and TACoApplication's RewardPaid has four parameters, I see two feasible solutions:

1. withdrawRewards function in TACoApplication contract emits the IApplication's RewardsWithdrawn event in addition to the currently emitted RewardPaid event.

2. Modify IApplication's RewardsWithdrawn event so the parameters are the same as RewardPaid (sender | stakingProvider | beneficiary | reward instead of stakingProvider | reward). This solution is probably less realistic since this will need to update deployed contracts like tBTC and RandomBeacon:

https://github.com/keep-network/keep-core/blob/7b46b2179e525e848ced5fe67445c109da8f3dfb/solidity/ecdsa/contracts/WalletRegistry.sol#L364

Related issue: threshold-network/solidity-contracts#156