JSON marshaler

[janisz]

Currently there is no json marshaller that is protojson compatible and does not relies on reflection. How about add it to VT proto.
Example implementation:

• perf(hackathon): MarshalJSON proto generator stackrox/stackrox#14418

[vmg]

I'm not opposed to merging this, but I'm not quite sure of the rationale. Usually when you're already working with jsonpb, performance is no longer a priority, so I don't foresee a lot of users for this functionality. The implementation in that PR seems to be a long way from fully passing the conformance suite (something which would be a hard requirement to merge it), but if somebody wants to pick it up, let's discuss!