Add interface to EvaluationClient (Go)

[zeryel]

Greetings. I want to start using Flipt, and I noticed that current SDK (Go) haven't interface for EvaluationClient - which makes it very difficult to write unit tests.
I suggest implementing the EvaluationClient's interface in SDK, i.e.:

type (
	// ClientInterface describes methods of EvaluationClient
	ClientInterface interface {
		EvaluateVariant(ctx context.Context, flagKey, entityID string, evalContext map[string]string) (*VariantEvaluationResponse, error)
		EvaluateBoolean(ctx context.Context, flagKey, entityID string, evalContext map[string]string) (*BooleanEvaluationResponse, error)
		EvaluateBatch(ctx context.Context, requests []*EvaluationRequest) (*BatchEvaluationResponse, error)
		ListFlags(ctx context.Context) ([]Flag, error)
		Close() error
	}

	// EvaluationClient wraps the functionality of making variant and boolean evaluation of Flipt feature flags.
	EvaluationClient struct {
		engine         unsafe.Pointer
		namespace      string
		url            string
		authentication any
		ref            string
		updateInterval int
		fetchMode      FetchMode
	}
)

// NewEvaluationClient constructs a Client.
func NewEvaluationClient(opts ...clientOption) (ClientInterface, error) {
    client := &EvaluationClient{
	namespace: "default",
    }
    ...
    return client, nil
}

[markphelps]

@zeryel yup we can definitely do that. Good call. We are about to bump the Go SDK version anyways so we can add this before that

On second thought, I opened up flipt-io/flipt-client-sdks#700 as I think there might be some discussions necessary on idiomatic Go and 'large' interfaces. We also don't have consistent interface usage across any of the Client SDKs (for languages that have interfaces) so maybe there's a larger discussion to be had around improving testability for the SDKs in general?

[zeryel]

I have read arguments in other discussion. Well, it does have its place to be, especially when we speak about SDK architecture in other languages. Anyway, I have a plan B - to create the necessary interfaces inside my application - so it's not such a big problem. Thanks for the feedback!