updated rebate to use native denom instead of reward denom

[sampocs]

Context

The rebate must now be issued using the native denom (DYDX) instead of the reward denom (USDC).

Brief Changelog

• Removed CalculateRewardsSplitBeforeRebate and CalculateRewardsSplitAfterRebate in favor of a simpler function CalculateRewardsSplit that returns the portion of rebate, stride fee, and reinvestment
• Updated withdrawal-rewards callback (the callback on the USDC query) to trade the full queried amount instead of checking for a rebate
• Updated withdrawal-host callback (the callback on the DYDX query) to optionally fund the community pool if a rebate was specified
• Updated unit tests

[riley-stride]

Looking solid to me - great work!

[asalzmann]

lgtm - although I'm slightly less familiar with this code, should get a review from @shellvish and @ethan-stride too!

[ethan-stride]

LGTM

One weird thing is that StrideCommission and RebateRate are not stored in the same format. We might also want to rename StrideCommission if it is really the percent of total fees not just the stride fees.

[shellvish]

This looks great, thanks a lot for doing this! Seems like this greatly simplifies the rebate logic!