Early initialization without Unreal CRC integration

[mujacica]

Since Sentry is initialized as an engine subsystem, there is a (small) period of time when the engine is initializing that it isn't covered by Sentry. Adding early initialization hook could solve this. To start with, customers would probably have to make engine modifications, until Epic accepts a deeper integration, but this could allow Sentry to provide a holistic solution for Unreal games without relying on any interactions with Unreal CRC.