UMIP 25 -Add GASETH-FEB21 and GASETH-MAR21 as supported DVM price identifiers

This UMIP will reference a synthetic token to be created with this price identifier. This token will be referred to as ‘uGAS’ and will represent the token that tracks this identifier with the most ETH volume on Uniswap unless a different contract is determined by voters to be more legitimate.

The DVM should support requests for a price that resolves to either the median monthly Ethereum gas price or a 2-hour Time-Weighted Average Price (TWAP) on the highest volume Uniswap ETH/uGAS pool. The price resolution method to use will depend on the the timestamp the price request was made at.