Expose `onCooldownStart` functionality as public API surface (support Aura Containers)

[ascott18]

The new WoW 12.1 Aura Container system's cooldown widgets can't be hooked through any means, so the existing auto-skin hooks do not work.

TullaCTC can skin these without trouble, though, if only addons were allowed to call the onCooldownStart hook to setup theming when they create their aura containers. I tried this myself with a quick hack and didn't see any issues - all customization seemed to apply as expected.

[Tuller]

Yes, this makes sense for aura stuff. A new registration method could also better help tag cooldowns too.

[Antorinia]

any updates on this feature?