Support pbjson emit_fields option

[EiffelFly]

Hi!

When generate the serde value with pbjson, this emit_vfields option is very handy when we don't want to emit the default value.

Really looks forward that protoc-gen-prost can support this. Does this lib has concern of implementing this option? If all things are good I could file a PR.

• Option to Emit Default Values When Serializing influxdata/pbjson#82 (comment)
• https://docs.rs/pbjson-build/latest/pbjson_build/struct.Builder.html#method.emit_fields