Generalize rewards-related methods and events for IApplication

[cygnusv]

In line with #88, I noticed there's no reference to rewards on IApplication, which implied that current prospective application contracts, namely PRE and tBTC, have their own implementation and interfaces. For the sake of interoperability with other dApps (e.g., dashboards), we should unify methods and events related to rewards and add them to IApplication.

To clarify, I'm not advocating for unifying the rewards distribution logic (although both PRE and tBTC follow the Unipool approach, a third application could follow a different rewards distribution mechanism), but for finding a generalized abstract interface that can be useful for all kinds of applications on Threshold. In very high-level terms, the functionalities of a Threshold application that are related to rewards are:

1. The Threshold protocol grants/pushes rewards to an application
2. The application has an internal mechanism to account for and distribute rewards to participating stakers
3. Stakers can query the amount of rewards they've accrued so far
4. Stakers can withdraw rewards from the application

I think all of these (except for functionality 2) should be generalized and added to IApplication.

[pdyraga]

Random Beacon application contract exposes function withdrawRewards(address stakingProvider) external. Identical function will soon be added to ECDSA (tBTC) application contract: keep-network/keep-core#2920. This should satisfy 4 assuming PRE uses something similar.

For 3, the call is trivial but we may have a problem proxying it through IApplication given the contract size constraints on RandomBeacon. We'll see what can be done.