Message#to_json should convert field names to lowerCamelCase

[aslakhellesoy]

According to the Protobuf 3 language guide:

By default proto3 JSON printer should convert the field name to lowerCamelCase and use that as the JSON name.

I know this library isn't currently targetting Proto 3, but is this something you'd be open to a PR for? Backwards compatibility could be maintained by making this an opt-in feature. For example:

json = message.to_json(lower_camel_case: true)

[film42]

Released in v3.10.3.