Add options to the protobuf processor to indent and emit unpopulated fields

[jkunnen-structurizit]

At this moment no indentation and no emit of unpopulated fields is possible in the protobuf processor when marshaling to json ouput.
Especially the option to emit unpopulated fields is needed.

[mihaitodor]

Hi @jkunnen-structurizit, can you please offer more details about this? I think the indentation issue can be solved using the format_json() bloblang method in a mapping processor, but I'm not sure what you mean by emitting unpopulated fields. Can you provide a simple example that I can run myself?