[PIP-028] Add Curve and Convex Receivers for fxUSD/ULTRA pool to receive PRISMA rewards

This proposal aims to add receivers for the fxUSD/ULTRA pool on both Curve and Convex. It will allow vePRISMA voters to direct PRISMA emissions to the pool once it is passes as a gauge and further grow TVL and liquidity for mkUSD. To accelerate this, f(x) will add the pool to the FXN gauge which will further incentivize it.

Read more: https://gov.prismafinance.com/t/fip-028-add-curve-and-convex-receivers-for-fxusd-ultra-pool-to-receive-prisma-rewards/148