Generic string to simulate stats amounts with durations and starting points

[Tierer]

Is your feature request related to a problem? Please describe.

Blizzard has been implementing a lot of stat bonuses built into cooldowns or legendary effects based on when an ability is cast. I always want to try and utilize them in the best way I can on PTR. Having a generic string where you can give a stat amount (percent or raw) with a set duration and when in combat to start it would be really helpful for a number of reasons. Firstly it would be something usable for a number of different buffs and effects and multipurpose. Secondly if the ability is getting tuned constantly, rather than having to rework an implemented string, if the string were already implemented for PTR, there would be no adjustment needed. Finally it could serve as a framework for strings when whatever buff you want to implement does hit live servers with final testing.

Describe the solution you'd like

Just some generic string that could be put into an APL, similar to how external_buffs.power_infusion="#" works.
Something like external_buffs.generic_stat="StatType/SpecifyPercentorRaw/Amount/Duration/StartTimes"

Describe alternatives you've considered

The way we simulated things without their implementation, such as warrior banner, were to use enchants and run a short sim to see relative gain per class, however you can only really test on pull with this.