Search: Add support for user-related filters introduced by WordPress 5.1

WordPress 5.1 is currently slated for release on February 21, 2019 and introduces some new filters around `WP_User_Query()` including `users_pre_query` and `pre_count_users`. Per [the announcement post](https://make.wordpress.org/core/2019/01/18/new-user-related-short-circuit-filters/), it looks like this this pattern will continue and additional filters will be introduced for other queries including short circuits for `WP_Comment_Query` and `WP_Site_Query`.

I wanted to kick off the discussion as to whether these filters will be integrated into Jetpack Search and compile some scenarios in which they would be useful.

Start with the issue's announcement post and the WordPress `WP_User_Query()` filters `users_pre_query` and `pre_count_users`. Review the possible Jetpack Search integration scenarios, including the mentioned future `WP_Comment_Query` and `WP_Site_Query` short circuits. Done would require a decided scope for integrating these filters, but the issue does not specify implementation files or tests.

Written by the indexing model from the issue text.