Search: Support configuring the order of date histogram aggregations

In JP Search, code / UI should support controlling the display order of Date Aggregations - sometimes you want to show dates in newest to oldest or vice versa, so the 'Filter' should support setting an `order` param (`asc|desc`) which controls that rendering.

Related code:

https://github.com/Automattic/jetpack/blob/e4477ebb9e13752a04776421351b7809039baf10/modules/search/class.jetpack-search.php#L1433-L1436

Start in modules/search/class.jetpack-search.php at the referenced date histogram aggregation code around lines 1433-1436, then trace how the Filter configuration reaches rendering. Done means the Filter accepts an order value of asc or desc and date aggregations display in the requested order.

Written by the indexing model from the issue text.