[Feature Flag]: Firebase Remote Config support

[bruno-garcia]

Firebase has Unity support: https://firebase.google.com/docs/remote-config/get-started?platform=unity

The goal is to capture the config/feature flag values for apps on Firebase, automatically.

Also: Add onboarding in Sentry. See:

• feat(flags): support Statsig webhook provider and secrets format sentry#85132
• feat(flags): add python onboarding sidebar sentry#80871

Depends on:

• [Feature Flag] API to track evaluations sentry-dotnet#4008

Relates to:

• [Epic] [Feature Flag] Firebase Support sentry#86039
• [Flutter] Feature Flags Reporting - Firebase Remote Config integration sentry-dart#2748
• [React Native] Feature Flags Reporting - Firebase Remote Config integration sentry-react-native#4591

Example/Reference PRs:

• feat(flags): add Statsig browser integration sentry-javascript#15319
• feat(flags): add Statsig integration sentry-python#4022