Support flat attributes (not only nested traits) on evaluation contexts

[danjuv]

When using this provider, to retrieve flags we need to pass attributes nested under traits, for example:

client.get_boolean_value(key, False, EvaluationContext(key, {"traits": {id: value}}))

This breaks automatic merging with OpenFeature transaction contexts and requires bespoke modifications to use this provider.

Ideally the provider would also accept flat attributes, for example:

client.get_boolean_value(key, False, EvaluationContext(key, {id: value})

This would help the provider conform more closely to OpenFeature standards while remaining backwards-compatible.