support $dateFromString operator for aggregation

[Andrei Popa]

Is your feature request related to a problem? Please describe.
It would be nice if we could support the $dateFromString operator. It would simplify the date conversion.

https://docs.mongodb.com/manual/reference/operator/aggregation/dateFromString/

[Joannis Orlandos]

Advocate of the devil here, but what prevents you from:

• Making a date locally?
• Using the operator in a "raw"/custom stage?

[Andrei Popa]

I was thinking of the ways I could speed up date conversion (Mongo native vs Vapor), but I admit I didn't read the raw docs :D