chore(tracing): extract app start to a standalone integration

[krystofwoldrich]

📢 Type of change

• Bugfix
• New feature
• Enhancement
• Refactoring

📜 Description

💡 Motivation and Context

💚 How did you test it?

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled
• All tests passing
• No breaking changes

🔮 Next steps

[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

- extract app start to a standalone integration ([#3852](https://github.com/getsentry/sentry-react-native/pull/3852))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description.

Generated by 🚫 dangerJS against 0b42472

[github-actions]

Android (legacy) Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	489.65 ms	523.85 ms	34.21 ms
Size	17.73 MiB	19.91 MiB	2.18 MiB

[github-actions]

iOS (legacy) Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1216.63 ms	1217.06 ms	0.43 ms
Size	2.36 MiB	3.01 MiB	666.41 KiB

[github-actions]

iOS (new) Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1232.34 ms	1233.98 ms	1.64 ms
Size	2.92 MiB	3.57 MiB	671.76 KiB

[github-actions]

Android (new) Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	344.73 ms	378.84 ms	34.11 ms
Size	7.15 MiB	8.19 MiB	1.04 MiB