Add `RewardsWithdrawn` event to `withdrawRewards` function in TACoApplication

[manumonti]

Type of PR:

• [ ] Bugfix
• [x] Feature
• [ ] Documentation
• [ ] Other

Required reviews:

• [ ] 1
• [X] 2
• [ ] 3

What this does: This will add RewardsWithdrawn to be emitted when staking rewards are withdrawn.

Issues fixed/closed: Fixes #275

Why it's needed: This event is included in IApplication interface, so it's intended to be emitted by all the Threshold applications when the function withdrawRewards() is called.

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

[manumonti]

TBD: tests