Report app start more granularly

[romtsn]

Description

The app start actually consists of multiple steps (e.g. process creation, bindApplication, etc.), so we could potentially create spans for each of this steps to give more insights for the users on where exactly can they optimize (e.g. system things like process creation cannot be optimized).

Prior art https://github.com/square/papa/blob/db3128e211a3cd916083d87d1d7eeac8cf8b8a56/papa/src/main/java/papa/internal/Perfs.kt

[philipphofmann]

On Cocoa, we already do that. Maybe worth having a look before starting to implement this https://docs.sentry.io/platforms/apple/performance/instrumentation/automatic-instrumentation/#app-start-instrumentation.

[adinauer]

Need to cache what happens before the SDK is initialized and later create spans with that information.

[markushi]

Consider adapting the app-start time here as well.