fix: Deduplicate trace generation during startup and scene loading

[bitsandfoxes]

Resolves #2235

Note: The flow differs from Editor vs actual build since not all RuntimeInitialization attributes get triggered.

In an actual build this is now the flow:

1. Init gets triggered by RuntimeInitialization SubsystemRegistration

   sentry-unity/package-dev/Runtime/SentryInitialization.cs

   Line 68 in dbbb63c

   SentrySdk.Init(options);

2. The SentryUnitySdk initializes the .NET SDK

   sentry-unity/src/Sentry.Unity/SentryUnitySdk.cs

   Line 38 in dbbb63c

   unitySdk._dotnetSdk = Sentry.SentrySdk.Init(options);

3. All integrations get registered
   • NEW The TraceGenerationIntegration refrains from regenerating any traces that would be covered by other integrations
4. The StartupTracingIntegration creates a transaction and it's child spans

   sentry-unity/src/Sentry.Unity/SentryUnitySdk.cs

   Line 42 in dbbb63c

   StartupTracingIntegration.StartTracing();

5. The first scene loads.
   • Note: For the first scene, the SceneManagerTracingIntegration is not getting invoked. This happens outside of the SceneManagerAPI.
   • This means there is no need to deduplicate between StartupTracing and SceneManagerTracing
6. For all subsequent scene loading, the SceneManagerTracingIntegration is solely responsible for generating a transaction. Finishing the transaction generates a new trace implicitely.

[bitsandfoxes]

@sentry review

[bitsandfoxes]

@sentry review