firebase-ios-sdk

Prepare for migrating FSTMutation to C++ Mutations (#3551)

* Finish porting Precondition to C++.

* Finish port of FieldTransform to C++

* Add ToString to all TransformOperations

* Rename Precondition::IsNone to is_none to match other is_type helpers

* Finish port of SnapshotVersion to C++

* Add is_type accessors to FieldValue

* Add nanopb::MakeStringView(const pb_bytes_array_t*)

* Remove DummyOperation from transform_operations_test