Modularize HMS into a separate module

[bbjay]

For privacy reasons, it would be very beneficial to be able to use OneSignal SDK without any HMS references.

This has come up a few times now: #1563 , #2307 , OneSignal/OneSignal-Flutter-SDK#868

Developers from OneSignal responded on two occasions with:

Longer term, we do want to modularize HMS into a separate module so you can include or exclude it without relying on workarounds. This isn’t something we’ll be able to ship soon, though—the change is fairly involved and we’re prioritizing stability and core improvements right now.

So I'm creating this issue as a feature request and to track progress on this.