Additional functionality for effects (Need Input)

[UmeboshiXI]

I'm working on a PR to modify some effect behaviors for future mobskill functionality and would like some input on how it would best be handled cleanly/future proof.

Some mob skills have decaying effects that we need to handle somehow.

Example: "Molluscous Mutation" used by Xzomits.

• Mob gains a defense percent boost that decays over time.
• From captures, it looks like the decay rate is not static like attribute down effects but is instead multiplicative.

Currently I'm using the Defense Boost subPower to handle the decay rate for testing purposes but there is also a special directional damage immunity mechanic already using subPower to calculate the angle for direction (See: a60f361).

• I'm not currently aware of any mobs that use decaying Defense Boosts + Directional immunity mechanic so it may be possible to
  split it based on if the effect has a tick rate > 0.

• Maybe we could find a way to use subType param?

• Or use a listener in a mixin?

• Or if we don't care for flexibility we could hardcode/gate the decay rate based on mob pool/family for Xzomits and special cases.