Set Firebase Remote Config flags on integration initialization

[kurtmarcink]

📜 Description

Calls Sentry.addFeatureFlag on initialization of SentryFirebaseRemoteConfigIntegration.

💡 Motivation and Context

Fixes #3007

💚 How did you test it?

flutter test firebase_remote_config/test/src/sentry_firebase_remote_config_integration_test.dart

📝 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
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

N/A

[buenaflor]

thanks for the contribution! lgtm but I'll let @denrase give the final approval

[buenaflor]

only missing piece is to add a changelog entry

### Features

- Set Firebase Remote Config flags on integration initialization ([#3008](https://github.com/getsentry/sentry-dart/pull/3008))

[denrase]

Just some smaller refactoring suggestions. Thank you for your contribution! 🙇