UMIP 93 -Add yvUSDC as a whitelisted collateral currency

This UMIP will add yvUSDC, an interest-bearing stablecoin tokenizing USDC deposited in YEARN V2 Vaults, as approved collateral currency. This will involve adding the currency to the whitelist and adding a flat final fee to charge per-request. The proposed final fee is 400 yvUSDC per request.