feat: feature flags

[stayallive]

This adds support for setting feature flag evaluations on the scope and spans. This is modeled after the Python SDK. Intended purpose is to allow us to integrate with Laravel Pennant.

See:

• https://develop.sentry.dev/sdk/expected-features/#feature-flags
• https://develop.sentry.dev/sdk/data-model/event-payloads/contexts/#feature-flag-context
• https://github.com/getsentry/sentry-python/blob/3e86962e267e86f51e8483c77dc1ad116f316d1c/sentry_sdk/feature_flags.py

[Litarnus]

Nice, looks good!