Support querying data for only selected beacons

[Siegrift]

It's common for searchers to only require values for certain beacons. Currently, they have to query them all and filter the ones they need from response. This requires a lot of data to be sent over the wire and causes delays/failures.

The disadvantage of this is that right now the endpoint is easily cache-able, because it's completely static. This would likely require some changes on CDN side (ideally the filtering would be performed by the CDN itself).