emit default values for non populated fields in protobuf

[ealonsodb]

When you use schema_registry_decode processor, the protobuf default values (0 for int32, "" for string, false for bool, 0.0 for float, etc...) are not populated in the row. With this new parameter you can have all those default values populated, no need to check for nulls after that processor

[CLAassistant]

All committers have signed the CLA.

[rockwotj]

Thanks for this! I will review after Thanksgiving.

One quick note is it would be good if the configuration option was either nested in a protobuf_options object or if the field was prefixed with protobuf to make it more clear what it was for (also for alignment with avro). I personally prefer the former option