Engine structure should have suffixes for controlling independent throttle

[JonnyOThan]

As a specific solution to the general problem in #2939, it would be convenient if engines provided access to the "toggle independent throttle" and "throttle percent" fields as top-level suffixes. Note these fields are normally only accessible when advanced tweakables are enabled.

[thexa4]

This is addressed in #2881

Let me know if you would like to help testing it out.