Distinct C++ classes must be introduced to represent feature flags that depend on the FCV and those that don’t:

• FeatureFlagBinaryCompatible - FCV independent feature flag
• FeatureFlagFCVGated - FCV dependent feature flag

The goal of this ticket is to create separate classes based on the current implementation of the FeatureFlag type, while preserving the exact same behaviour for both FeatureFlagFCVGated and FeatureFlagBinaryCompatible.
Note that the usage of the already available FeatureFlag class should not be adjusted as part of this task.