[Convex] Introduce cvxFXS single-sided staking

Summary

• Move the 10% Convex Frax protocol fee from the cvxFXS-FXS curve liquidity pool to a single sided staking cvxFXS contract.
• Move the veFXS fees from the cvxFXS-FXS curve liquidity pool to single sided cvxFXS staking contract.

Abstract

When Convex Finance first introduced cvxFXS, the Convex Frax Boosting platform did not yet exist. Since there were no Convex fees, Convex decided to launch with all rewards directed at the LP pool. This also allowed cvxFXS to receive CVX rewards via the minting mechanic. After the boosting platform was launched, rewards continued to all be pushed to the LP pool.

Now that the boost platform is alive and running well, this proposal will authorize the creation of a single sided cvxFXS staking contract which will receive both Convex fees as well as veFXS fees, of which are currently being distributed to the LP pool. This will also give cvxFXS holders more options on how they want to hold and use their cvxFXS positions.