add Unwrap method to allow to detect root cause error with errors.Is

[C-Pro]

We realised that detecting certain types of errors does not work as expected because error types are missing Unwrap method required for errors.Is to work:

	sub.OnError(func(e centrifuge.SubscriptionErrorEvent) {
		if errors.Is(e.Error, centrifuge.ErrUnauthorized) {
			// We never got here, even though Unauthorized error is logged.
		}
		log.Error(e.Error)
	})

This PR adds corresponding methods to error types in errors.go and a corresponding test (fails without Unwrap, but works with it in place). I suspect it may be not required for some types like ConfigError, but adding the method anyway for uniformity.

Also changed a --client_insecure to --client.insecure in the workflow as old flag no longer works with latest centrifuge (this leads to workflows failing).

[C-Pro]

Actually there's a workaround: since Err field is exported we can do this:

	sub.OnError(func(e centrifuge.SubscriptionErrorEvent) {
		if subSubError, ok := ev.Error.(centrifuge.SubscriptionSubscribeError); ok {
			if errors.Is(subSubError.Err, centrifuge.Timeout) {
				// do something
			}
		}
		log.Error(e.Error)
	})

But I think directly using errors.Is on the returned error is nicer.

[FZambia]

Great! Many thanks for the improvement and CI fix