Add Client Event Observation

Add event observation methods to the Client interface for parity with `OpenFeatureAPI.observe()`.

**Specification References:**
- [5.2.1](https://openfeature.dev/specification/sections/events#requirement-521): The client MUST provide a function for associating handler functions with a particular provider event type.

**Implementation Notes:**
- Add `Client.observe() -> AnyPublisher` method
- Currently only `OpenFeatureAPI.shared.observe()` exists
- Client-level observation should filter to events from the client's associated provider

Start by locating the Client interface and the existing OpenFeatureAPI.shared.observe() implementation. Add the client-level observation API described in the issue, ensuring it only emits events from the client’s associated provider; completion means the behavior matches specification 5.2.1 and the stated publisher signature.

Written by the indexing model from the issue text.