feat: initialize client core and ping endpoint

establishes the foundational architecture for the Onspring Go SDK.

changes included:
- add `Client` struct and `NewClient` constructor with authentication.
- implement functional options pattern (WithHTTPClient, WithBaseURL, etc.).
- add `OnspringAPIError` type for handling non-2xx responses.
- implement internal request construction and execution logic.
- add `Ping` service for API health check verification.
- add comprehensive unit tests and mock server infrastructure.

option_test.go

@@ -0,0 +1,54 @@
+package onspring_test
+
+import (
+	"net/http"
+	"testing"
+	"time"
+
+	"github.com/StevanFreeborn/onspring-api-sdk-go"
+)
+
+func TestWithHTTPClient(t *testing.T) {
+	t.Run("it should set a custom HTTP client on the Onsring client", func(t *testing.T) {
+		customHTTPClient := &http.Client{Timeout: 10 * time.Second}
+
+		clientWithCustomHTTP := onspring.NewClient(
+			"test-api-key",
+			onspring.WithHTTPClient(customHTTPClient),
+		)
+
+		if clientWithCustomHTTP.HTTPClient() != customHTTPClient {
+			t.Errorf("Expected custom HTTP client to be set")
+		}
+	})
+}
+
+func TestWithBaseURL(t *testing.T) {
+	t.Run("it should set a custom base URL on the Onsring client", func(t *testing.T) {
+		customBaseURL := "https://custom.onspring.com"
+
+		clientWithCustomURL := onspring.NewClient(
+			"test-api-key",
+			onspring.WithBaseURL(customBaseURL),
+		)
+
+		if clientWithCustomURL.BaseURL() != customBaseURL {
+			t.Errorf("Expected base URL to be %s, got %s", customBaseURL, clientWithCustomURL.BaseURL())
+		}
+	})
+}
+
+func TestWithAPIVersion(t *testing.T) {
+	t.Run("it should set a custom API version on the Onsring client", func(t *testing.T) {
+		customVersion := "3.0"
+
+		clientWithCustomVersion := onspring.NewClient(
+			"test-api-key",
+			onspring.WithAPIVersion(customVersion),
+		)
+
+		if clientWithCustomVersion.APIVersion() != customVersion {
+			t.Errorf("Expected API version to be %s, got %s", customVersion, clientWithCustomVersion.APIVersion())
+		}
+	})
+}