Consider upgrading provisions to support Warmers as well as Hooks

[whitfin]

Right now provisions are only available on hooks, which is odd because warmers feel like they should also support them.

We should look into adding support on warmers to ensure the cache is always in sync.

[whitfin]

After looking at this further I believe the provisioning should be pulled out of hooks into a generic behaviour and module to handle provisioning for anything relevant in a cache (which will be hooks + warmers on first implementation).

This is technically not a breaking change, and I doubt many people are actually using this functionality that much anyway, but it's big enough that I'd rather do it in the next major.

I thought about stripping out provisions completely, because there's literally only one, but the implementation is so straightforward that it's not hurting anything even if the abstraction isn't entirely useful.