[Feature Flag]: Firebase Remote Config support

[bruno-garcia]

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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:

• https://github.com/getsentry/sentry/pull/85132
• https://github.com/getsentry/sentry/pull/80871

Depends on:

• https://github.com/getsentry/sentry-dotnet/issues/4008

Relates to:

• https://github.com/getsentry/sentry/issues/86039
• https://github.com/getsentry/sentry-dart/issues/2748
• https://github.com/getsentry/sentry-react-native/issues/4591

Example/Reference PRs:

• https://github.com/getsentry/sentry-javascript/pull/15319
• https://github.com/getsentry/sentry-python/pull/4022