Move dataset_ids to separate message

[mbaudis]

the current BeaconAlleleResponse message embeds the optional specification of dataset ids. With a foreseeable move to multiple variant and metadata message query types, it seems logical to have the datasets specified outside of the variant queries.

[jrambla]

I agree. But in the future... As it is an integral part of the request, I don't clearly see how to do it. Also, as these are just two "small" parameters, I suggest to go ahead duplicating them in both requests and refine it when we see how and see it fit.

[mcupak]

What would this look like encoded in proto3?