Set Firebase Remote Config flags on integration initialization

[kurtmarcink]

:scroll: Description

Calls Sentry.addFeatureFlag on initialization of SentryFirebaseRemoteConfigIntegration.

:bulb: Motivation and Context

Fixes #3007

:green_heart: How did you test it?

flutter test firebase_remote_config/test/src/sentry_firebase_remote_config_integration_test.dart

:pencil: Checklist

• [x] I reviewed submitted code
• [x] I added tests to verify changes
• [x] No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• [x] I updated the docs if needed
• [x] All tests passing
• [x] No breaking changes

:crystal_ball: Next steps

N/A

[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))

[kurtmarcink]

LGTM! 🥳 We only need to sync with main and add the cahngelog entry, if not already there.

@denrase Done!