Flag metadata implementation

[thomaspoignant]

As mentioned in the specification :

If the flag metadata field in the flag resolution structure returned by the configured provider is set, the evaluation details structure's flag metadata field MUST contain that value. Otherwise, it MUST contain an empty record.

This flag metadata field is intended as a mechanism for providers to surface additional information about a feature flag (or its evaluation) beyond what is defined within the OpenFeature spec itself. The primary consumer of this information is a provider-specific hook.

The actual version of the SDK does not allow the providers to return flag metadata.
I guess it should be an extra field in the ProviderEvaluation and it should be forwarded in the FlagEvaluationDetails.