UMIP 66 -Add USDBTC_18DEC, BCHNBTC_18DEC, ELASTIC_STABLESPREAD/USDC_18DEC, STABLESPREAD/USDC_18DEC, STABLESPREAD/BTC_18DEC and AMPLUSD_18DEC as supported price identifiers

This UMIP proposes adding USDBTC_18DEC, BCHNBTC_18DEC, ELASTIC_STABLESPREAD/USDC_18DEC, STABLESPREAD/USDC_18DEC, STABLESPREAD/BTC_18DEC, AMPLUSD_18DEC as supported price identifiers.