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.

[linear]

UNREAL-93 Early initialization without Unreal CRC integration

[nwhite-riot]

My thought process here is that Sentry could have its own FCrashReportingThread that acts as an initialization/hook point for Sentry, and the Sentry subsystem becomes the runtime access to the underlying Sentry SDK implementation. It would mean that the subsystem is no longer responsible for initialization (or if it is, code may have to change to make it available in early engine initialization).

Ideally, we could "productionalize" the functionality and then propose it to Epic for upstreaming.