[Feature Request] Ability to include section headers in animation

[zackdotcomputer]

I'm currently encountering an issue trying to use Spruce in an app where the animations don't "look right" when used on a UITableView that includes section headers because the section headers will appear suddenly at the beginning of the animation rather than being faded in with their cells.

Ideally, we'd implement a solution like the one that @Vyax started on in this possibly abandoned PR from 2017. If necessary, I'm happy to take point on updating the PR to merge cleanly with 2.10 base and add this functionality.