Allow targeting specific fields for `@updatedAt`

[sanny-io]

The @updatedAt attribute updates the timestamp when any change is made to a record. I propose adding a new fields parameter, which only updates the timestamp when any of the fields specified are changed.

Prisma issues: prisma/prisma#7602 prisma/prisma#12164

Sample Usage

model User {
  name String
  nameUpdatedAt DateTime @updatedAt(fields: [name])
}

I will start implementing this feature if and when #572 is merged, but these features would pretty much be mutually exclusive.