* add a new property `timestampsInSnapshotsEnabled` to `FirestoreSettings`, `false` by default;
* add a verbose warning message urging users to opt into the new behavior;
* set `timestampsInSnapshotsEnabled` to true in the integration tests to reduce the verbose console
spam during the test run and make sure the flag won't break anything once it's flipped.
* Expose network management in public API
* Clean up a few more references to the internal access of network management
* Move test
* Update comments
* Swap _Nullable for nullable
* Fix comment
* Add tests, including swift
* Styling