feat: Update observeEvents() API

[fabriziodemaria]

This PR

Update the OpenFeatureAPI.observeEvents() API:

• It can be called by the Application layer before any provider is configured, and it will still receive events once a new provider is configured and emitting events
  • Thanks to this, we can now recommend setting up the observation before setProvider is called, thus ensuring no event from the configured provider is lost due to raciness between observer and setProvider

Usage example (from Application layer):

viewModelScope.launch {
    OpenFeatureAPI.observeEvents().collect {
        println(">> Event received: $it")
    }
}

viewModelScope.launch {
    OpenFeatureAPI.setProviderAndWait(
        ConfidenceFeatureProvider.create(
            app.applicationContext,
            clientSecret
        ),
        Dispatchers.IO,
        ctx
    )
}

[fabriziodemaria]

Note to self: shall we close the flow on shutdown? Shall we emit something else on clearProvider?