[FEATURE] Implement events in multi-provider

[askpt]

Requirements

Implement event support in the multi-provider in alignment with the OpenFeature events concept (see https://openfeature.dev/docs/reference/concepts/events).

Requirements:

• Integrate event handling capabilities so that the multi-provider can emit and handle relevant events as defined by the OpenFeature specification.
• Ensure that events such as provider state changes, flag evaluations, and errors are surfaced according to the conceptual documentation linked above.
• Add appropriate hooks for consumers to subscribe to and react to these events.
• Include documentation and usage examples for the new event features.

Reference: https://openfeature.dev/docs/reference/concepts/events