JSON printing / parsing of `optional` fields

[agasparovic]

Now that scalar field presence is supported in proto3 (hooray! 🎉), it should be possible for JSON printing and parsing to take advantage of it:

• Parsing: When a field defined as optional is provided in JSON input (even if its value is the default), parsing into a proto message should result in the field being set.
• Printing: When a field defined as optional is set in a proto message (even if its value is the default), it should be included in the JSON output when printing.

It may make sense to guard this new functionality behind a parameter like always_include_optionals_when_set in JsonParseOptions and JsonPrintOptions.

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