Make CollectionReference implement Query

[darinf]

Given that C++ types are exposed as structs and structs do not support inheritance in Swift, the fact that the C++ CollectionReference inherits from Query is not exposed on the Swift side. We then workaround that by defining a protocol that both Query and CollectionReference can conform to.

Also expose CollectionReference.path() and FieldValue.serverTimestamp().

[brianmichel]

Works for me, this kind of a gnarly problem that I assume we might run into again, it would be rad if the toolchain helped us more here in the future.

[darinf]

Works for me, this kind of a gnarly problem that I assume we might run into again, it would be rad if the toolchain helped us more here in the future.

I think we should probably just switch over to wrapping all of the C++ types with Swift class types. Then we can have inheritance and other nice things.