Search across all active variant prices

[jaroslavtyc]

Our shops work primarily with multi-variant products.

Those variants are often priced differently each other.

And we need to search by those different-per-variant prices.

This proposal saves to Elastic all unique variant prices. Thanks to Elastic behavior that every field is an array in fact we do not need to do any other changes - no index change / rebuild, no search query changes.

My skill is insufficient to update Behat tests to test those changes, please add them by your own.