Is your feature request related to a problem? Please describe.
Sometimes I want to apply a custom filter in the aura trigger, like check if the aura is crowd control by checking if it's spellID is in DRData library, or check if aura passes a set of my own conditions, for example if CompactUnitFrame_UtilShouldDisplayBuff(...) function returns true for this aura.
Right now to achieve this I have to create a custom event trigger, but it would be really nice to have an additional filter in Aura trigger.

Describe the solution you'd like
In Aura trigger I'd like to see an additional filter "Custom Filter" or "Custom Function", which, once enabled, will allow us to write a custom function like:
function(spellId, ...) return spellId == 123; end
Basically it should run this function and pass aura information as argument(s). If the function return true - trigger will proceed with other conditions/filters. If returns false - it won't trigger.
Also I think this filter should be checked last, after all standard ones.

That's mostly a duplicate of #937, feel free to add to that ticket.