Add options to protobuf processor: emit_unpopulated and emit_default_values

[aolwas]

This PR implements #2506 as an extension of what was done in #2219

Not a GO expert, hope I've done everything's right.

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[KernelPanicAUS]

hey @aolwas, do you need any help with this one?

[aolwas]

hey @aolwas, do you need any help with this one?

HI @KernelPanicAUS

Since I opened this MR, I chose another tool for my processing pipeline, so I don't really plan to work on this MR anymore.

However it seems still an interesting improvement, so feel free to take the ownership of the MR or just close it as you prefer.