Promote sandbox facets to the main facets module

Last year we created functionality to compute facets during hits collection. As the functionality is experimental, we added it to [the sandbox module](https://github.com/apache/lucene/blob/main/lucene/sandbox/src/java/org/apache/lucene/sandbox/facet/package-info.java).

We want to move the code from the sandbox to the main facets module, as a separate package.

This is a "parent" issue to discuss / track missing pieces before the promotion can happen, as well as the promotion itself. To make it easier to discuss subtasks, I've created a [google doc](https://docs.google.com/document/d/1PF9KWYboy6terrPp8Frizlkp1ee09RX-DsuZrBux-Oo/edit?usp=sharing). The doc consists of 3 parts: promotion from the sandbox, making it feature complete, and follow up tasks. This github issue is for the first part only.

Start with the sandbox package-info.java linked in the issue and review the promotion section of the linked Google document. Trace the existing hit-collection facet functionality and identify the missing promotion pieces before moving it into the main facets module as a separate package. Done means the sandbox implementation has been promoted and the tracked promotion items are addressed.

Written by the indexing model from the issue text.