Add more marshaler options for ProtoJson converter

[vbmithr]

What was changed

I added options to be passed to marshaler code for Proto/JSON.

Why?

Automatic OCaml deserialization of JSON-encoded Protobuf does not work if empty (nil) fields are not present.

Checklist

1. Closes

2. How was this tested:

I observed in my own code the expected result, i.e. JSON-encoded Protobuf now contains nil fields.

3. Any docs updates needed?

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[cretz]

LGTM, will let @Quinn-With-Two-Ns look

[cretz]

Thanks for the contribution!