[Feature Flag] API to track evaluations

### Problem Statement

This request is agnostic of integrations.

tl;dr: Add new API: `AddFeatureFlag` to SDK

Develop docs:
- https://develop.sentry.dev/backend/application-domains/feature-flags/

### Solution Brainstorm

This will unblock Unity. We'll be adding support for Firebase, Unity Cloud Services and other providers in the future.

But is also relevant for .NET, for the current supported providers (e.g: [LaunchDarkly](https://nugettrends.com/packages?months=72&ids=LaunchDarkly.EventSource)) but also, [Azure App Configuration](https://azure.microsoft.com/en-us/products/app-configuration):