[Flutter] Feature Flags Reporting - Firebase Remote Config integration

[kahest]

Description

This is to add an integration into the SDK to automatically detect and report updates to feature flag configurations (evaluation tracking) performed in the app via Firebase Remote Config: https://pub.dev/packages/firebase_remote_config

Dev docs on feature flags integrations: https://develop.sentry.dev/sdk/expected-features/#integrations
Existing integrations (product): https://docs.sentry.io/organization/integrations/feature-flag/
Existing integrations with evaluation tracking (JS): https://docs.sentry.io/platforms/javascript/feature-flags/#enable-evaluation-tracking