Protobuf processor options: EmitUnpopulated & EmitDefaultValutes

[aolwas]

Hello blobs,

In my team, we are currently evaluating benthos to replace an inhouse tool used to deserialized and store protobuf messages in S3.
Our PoC went well until we found we couldn't set the processor to export default-valued fields to be retro-compatible with our post-processing pipelines.

Since the option is available in google protobuf go module (cf package doc), it would be great to expose it in benthos as done for other ones in #2219.

Not a Go expert but I already have a PR ready to submit.
Hoping to have this feature integrated to really enjoy this nice piece of software.