Support for tracking feature flags

## Describe the idea

Support for Sentry's feature flags in Ruby SDK.

Background: https://github.com/getsentry/sentry/discussions/73542#discussioncomment-11671314

## Why do you think it's beneficial to most of the users

> Enabling a feature flag integration provides deep insights into the state of your application prior to an error. A list of flags and their evaluation results are displayed on each error event in the order of evaluation. Integrating Sentry with your feature flag provider enables Sentry to correlate feature flag changes with new error events and mark certain changes as suspicious.
>
> -- https://docs.sentry.io/product/issues/issue-details/feature-flags/

## Possible implementation

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