Add a way to change entity max health

This small "API" adds the ability to change the max health of entities.

This could be used for mods to modify the difficulty or to change the health ratios.
I would like to use this in my own mod to change player health to 100 and also change mob health.

I have tested it a bit, not 100% yet and hopefully I didn't do something wrong with the commit and pull request process, this is my first time.