Port the `CJsonWriter` utility class from upstream, which makes outputting correct JSON easier.
Add `CJsonWriter` as an abstract class that can write to different outputs. Two implementations `CJsonFileWriter` (writes to a file) and `CJsonStringWriter` (writes to an `std::string`) are added. Upstream `CJsonWriter` can only write to files.
The same tests are added for both implementations. Duplicate code is avoided by using typed tests with two separate test fixtures.
