feat: support multiple PostHog instances part II

[ioannisj]

💡 Motivation and Context

Second part of supporting multiple instances

Todo:

• Now storage is per api key, with some old storage migration logic so we don't miss any events. Old events will be migrated to the first API used
• Maintain one instance per api key. So, calling .with() with the same API key should return the same PostHogSDK instance
• Replay and element autocapture integrations installed only on first created instance
• captureApplicationLifecycleEvents (autocapture) only on first created instance
• captureScreenViews only on first created instance
• Decouple URLSessionInterceptor from PostHogSDK.shared
• Decouple UIApplication.sendEvent from PostHogSDK.shared
• Decouple SwiftUI postHogScreenView from PostHogSDK.shared
• Decouple URLSession extensions (replay network logs) from PostHogSDK.shared

#skip-changelog

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

[marandaneto]

I think to avoid this we can do this:
https://github.com/PostHog/posthog-android/blob/41ac8e9dbf989348823052c8c0f14c9c0d1fa2fd/posthog/src/main/java/com/posthog/PostHog.kt#L125-L131
we create the instances, and "install" them once the SDK is operational at the end of the setup method

[ioannisj]

Refactored to add PostHogIntegration. To be used only internally but should help as we continue adding integrations

[marandaneto]

@ioannisj for URLSessionExtension you can just add an optional param to the methods, with a posthog instance, and use the shared instance if the param is null/not given
if the extension can be used in objc, we need a new overload, though

[marandaneto]

@ioannisj for URLSessionExtension you can just add an optional param to the methods, with a posthog instance, and use the shared instance if the param is null/not given if the extension can be used in objc, we need a new overload, though

same for postHogScreenView extension

[marandaneto]

left a question otherwise LGTM