Make shields absorb ACU explosions

[lL1l1]

Description of the proposed changes

Resolves #5006. Gives shields the ability to absorb ACU explosions. The absorption behavior can be easily configured in the new ShieldAbsorptionValues.lua file, similar to how armor types are set up.

Testing done on the proposed changes

Spawn power, a shield, and an engineer under the shield. Spawn an enemy/friendly ACU outside the shield and explode it. The shield should take 2500 damage and everything underneath will stay unharmed.

Checklist

• [x] Changes are annotated, including comments where useful
• [ ] Changes are documented in the changelog for the next game version

[Garanas]

@lL1l1 this is quite an innovative solution, much better than what I would've made of it 😃 !