Support dates in raw queries

[pimeys]

Addresses issue prisma/prisma#2533

This is the engines part of the issue and needs support from the client. To allow querying with dates, the user should pass a date object in the query, which client coerces into a JSON object, marking the type correctly:

{
  "prisma__type": "date",
  "prisma__value": "1996-12-19T16:39:57+00:00"
}

This gets then parsed as a DateTime, allowing its usage in the query parameters.

[tomhoule]

lgtm 👍