UMIP 31-Add STABLESPREAD/USDC and STABLESPREAD/BTC as price identifiers (DEPRECIATED)

These price identifiers have been deprecated. It is highly recommended that contract deployers do not use these identifiers in their current state for new contracts. Price requests for this identifier from contracts created after 02/26/21 00:00 UTC will likely not be resolved correctly. Price requests from contracts created before this time will not be impacted.

The DVM should support price requests for the STABLESPREAD/USDC and STABLESPREAD/BTC price indices, denominated in USDC for STABLESPREAD/USDC and in BTC for STABLESPREAD/BTC. These indices are 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.