RFC: Track total rewards earned per contract

[chris-ricketts]

It would be incredibly useful to have a query like TotalContractRewards, which returns the total rewards generated by a contract over it's lifetime (regardless of rewards recipient address changes).

From the perspective of interacting with the rewards module from CosmWasm contracts, it would allow a contract to be the rewards recipient of multiple contracts and know how much each contract contributed it's total earnings.

This is not possible currently as the rewards contract can only query it's total outstanding rewards from all nominator contracts.

The addition of the TotalContractRewards query would allow the following flow:

1. Nominator Contract (NC) sets both it's owner and rewards address to be the Rewards Contract (RC). Notifies RC and specifies a collector.
2. RC then verifies owner and rewards addresses before querying NC's TotalContractRewards, storing the result.
3. Time goes by, rewards are collected from all NCs
4. NC collector comes to collect something based on generated rewards, executes Collect on RC.
5. RC queries current TotalContractRewards comparing it to the original stored value, calculates how much something to give to collector.

The current workaround is for the RC to create a child contract per NC to act as the rewards recipient. As well as being both less efficient gas and lines of code wise, it also degrades UX by forcing collections to be made on a per NC basis.

[zanicar]

Moved to Discussions under Ideas as this is an RFC;