feat: Implement FieldValue.minimum() and FieldValue.maximum()

[brettwillis]

Taking a bit of a liberty here to implement something that touches the public API, but I figure it's simple enough and it's been mentioned that it is on the list but not a high priority, so hopefully this is a help an not a hinderance 🙏🏼

Fixes #1701

[brettwillis]

Also an off-topic request (it is a backend feature not an SDK feature): It would be create if the backend also supported Timestamps as min/max operands, not just numbers. Would be super useful for incrementing/retaining "createdAt"/"updatedAt" timestamp fields, as an example.

[MarkDuckworth]

Thanks @brettwillis. At a glance this looks good. I'll do a full review soon. We will have to hold off on releasing this until after April 11th.

[MarkDuckworth]

Update on this. Since this is a public API change, and one that will also be ported to all Firebase/Firestore SDKs, I have initiated an internal API review for the change.

Also, we're considering alternative names takeMinimum(val) and takeMaximum(val), to give the methods verbs and make them consistent with other FieldValues.

[brettwillis]

Ok no worries thanks for the update.