Map UUID keys as GraphQL "ID" type

Currently, Hasura creates a new scalar type called `uuid` for UUID keys. Because it's a non-standard type, using it with clients usually means jumping through some hoops to treat the value as a string. But, that's really all the standard `ID` type does anyway, as far as I know. I think if this type were remapped in the schema, it'd improve the ergonomics of working with a Hasura-based GraphQL server.