Refactor FTargetEntityFragment so that we don't have to repeat default values twice for cover related variables

[Leroy231]

Do this on top of https://github.com/HaywireInteractive/OnAllFronts/pull/351.

• [x] Move all variables we reset in FTargetEntityFragment::UnclaimCover into a new FMassCoverFragment.
• [x] Add FCoverFragment in the trait that adds FTargetEntityFragment to entities.
• [x] Move FTargetEntityFragment::UnclaimCover onto FMassCoverFragment. This will require updating FTargetEntityFragment::Reset to take in a reference to FMassCoverFragment so it can call FMassCoverFragment::UnclaimCover.