No method to retrieve(or set in builder) increase in exhaustion from DamageSource

[parlough]

In Minecraft DamageSources can cause an increase in exhaustion in the attacked player, but this amount does not seem to be accessible from Sponge's DamageSource. I think it should even be possible to set this with the builder as well.

Edit: So this actually increases exhaustion, so MCP's name of hunger damage is a little misconceiving. See attached picture:

[ryantheleach]

@Meronat I've assigned this to you, as you already have commits in your SpongeAPI I presume there is a PR somewhere pending?

[parlough]

#1674 will solve this is you want to review it :)