feat: Introduce CacheClearer to force-clear static caches

[robmaceachern]

Allows clients to force-clear the static caches used in Blueprint. This intended as a stop-gap solution (famous last words, I know) until work is done to scope the cache lifetimes more appropriately.

[kyleve]

Do we need to clear out / replace any static prototype views as well? (Eg, in AttributedLabel?)

[robmaceachern]

(commenting here for threading purposes)

@kyleve:

Do we need to clear out / replace any static prototype views as well? (Eg, in AttributedLabel?)

Great question! When I was discussing this with @watt last week we both agreed that the AttributedLabel prototype view is probably ok as-is. It should be updated frequently enough in regular usage that even in the unlikely case it is momentarily retaining something somehow (I guess through a a11y closure?) it'll get freed up pretty quickly.

Happy to deal with it too, though! If it was in the same module I'd just reset the prototype but I can't reach it from BlueprintUI.

If we end up exposing a wrapper in Market for these cache clearing things we could always just measure an empty label there too.

[johnnewman-square]

Nice! Providing clients the ability to manually clear the cache feels like a good stop-gap that we can build on top of going forward.