Add validation rules to payload_data

[chuwy]

@alexanderdean do you think it has some value? I think at least it can provide maningful error messages from SCE if someone made mistake in tracker implementation.

For example tnuid is always UUID, br_features_flash is always 1 or 0.

Also I think many our schemas like this can be enhanced by adding description property, as many keys are pretty cryptic.

[alexanderdean]

Hey @chuwy - it's an interesting idea. I don't know how much we should be investing in making the Snowplow Tracker Protocol stricter, versus just moving away from it towards the new model (graph of self-describing JSONs)...