Moves network request instrumentation initialization to a bg thread

[mghaznav]

Summary

Moves URLSessionInstrumentation initialization off the main thread onto a detached background task so the Objective-C runtime class scan does not block FirebaseApp.configure().

Changes

• Initialize URLSessionInstrumentation asynchronously on a background thread with ThreadHelper.isNotMainThread() verification.
• Capture self weakly in spanCustomization and make filterNetworkRequests static to avoid a retain cycle with URLSessionInstrumentation.
• Allow localhost network requests in filterNetworkRequests (only filtering out firebasetelemetry requests).
• Add unit tests in FirebaseCrashlyticsTelemetryTests.

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[mghaznav]

/gemini review

[danger-firebase-ios]

	1 Warning
⚠️	Did you forget to add a changelog entry? (Add #no-changelog to the PR description to silence this warning.)

Generated by 🚫 Danger

[gemini-code-assist]

Code Review

This pull request refactors FirebaseCrashlyticsTelemetry to initialize URLSessionInstrumentation on a background thread and adds thread-safety locks around shared resources. It also introduces unit tests for network request filtering and span customization. Feedback recommends optimizing the network request filter to avoid string allocations and safely handling potential nil values in both the filter and test teardown.