UMIP 121 -Add TOKEN_PRICE as a supported price identifier

The DVM should support price requests for TOKEN_PRICE identifier that returns a price of any fungible token expressed in units of another fungible token, cryptocurrency or fiat currency. Ancillary data will guide voters in identifying which token should be priced, what should be its quote currency, which markets should be queried and any other processing instructions for resolving a particular price request.

This UMIP outlines the critical considerations for any user of this identifier. The user acknowledges that extra steps are required to use the identifier for their specific TOKEN_PRICE to resolve at expiry.