[React Native] Feature Flags Reporting

[kahest]

Description

This is to track progress towards initial support of Feature Flags Reporting.

Dev docs: https://develop.sentry.dev/sdk/expected-features/#feature-flags

Product docs: https://docs.sentry.io/product/issues/issue-details/#feature-flags

In scope:

• API to report feature flag states to the SDK
• integration with Firebase Remote Config

Out of scope:

• work outside of the SDK (covered by different team)
• other integrations