JSON output lacks add_whitespace and always_print_primitive_fields

[jiawen]

My projects serializes proto3 messages to JSON as a nice human-readable format that also allows for some light editing. However, the current Swift output is very hard to read, as everything is on one line.

The C++ API has add_whitespace and always_print_primitive_fields options that the Swift version lacks.

It seems reasonably easy to add and test.

[tbkka]

Reformatting JSON is pretty easy to do. For example, you can easily invoke JSONSerialization from Swift Foundation to deserialize arbitrary JSON and re-serialize in a prettier format. So I'm not convinced we need to add pretty-printing to our JSON encoder.
I'm not as familiar with always_print_primitive_fields; that might be an appropriate addition.

[jiawen]

Makes sense. TIL JSONSerialization.

The idea behind always_print_primitive_fields is also discussed here #861. I would also appreciate to have this option!

Best, Jeff