UMIP 28 -Add STABLESPREAD as a price identifier

The DVM should support price requests for the STABLESPREAD price index, denonominated in ETH. STABLESPREAD is defined as: min(max(A - B + 1, 0), 2) , where A refers to an equally weighted basket of {UST, CUSD, BUSD}. B is an equally weighted basket of {USDC, USDT}. UST is TerraUSD, the interchain stablecoin connected by the Cosmos IBC. CUSD is Celo Dollar, the stablecoin on the Celo network. BUSD is Binance USD, a stablecoin issued by Paxos in partnership with Binance. USDC is a centralized stablecoin issued by Circle and Coinbase. USDT is a non-transparent centralized stablecoin issued by Tether, currently the most used stablecoin on Ethereum.