Migrate to proto3

The absence of certain fields when they have their default value makes it seen, like FP switched to
 proto3. This just removed the required annotation on each field. Also: Enums are required to be 0-based. I wasnt sure how facepunch is handling this, so i for now introduced a RESERVED enum value for AppEntityType and EntityType. This is not tested yet, but only an early fix to show whats broken in general.

Author: JulianMa