chore: Move startup tracing into the SDK

[bitsandfoxes]

This is in part to help with #2227

We can have startup tracing be its own integration within the SDK instead of shipping it as a .cs that compiles with the game.

Things to note:

1. We can use the RuntimeInitializeOnLoadMethod attribute from within the SDK. The only requirement is that those are static methods and take no arguments
2. These methods get called whether the integration got registered or not, hence the IsStartupTracingAllowed check.
3. For each method we have to first confirm that it is "allowed" to create a span
4. We make sure this only ever gets executed once by setting the StartupAlreadyCaptured after scene loading.
5. Setting the StartupAlreadyCaptured also handles the case of manual initialization later than during startup since
   • Those methods always get called
   • We can make sure that late initialization does not (wrongly) start a new startup transaction

#skip-changelog

[bitsandfoxes]

We didn't have any tests on this prior. :(

[bruno-garcia]

How much of it was AI?

[bitsandfoxes]

Instead of having Startup Tracing be split between SentryInitialization and SentryIntegration we can delete the latter and move everything into internal StartupTracing inside the SDK.

[bruno-garcia]

How much of it was AI?