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.go

@@ -0,0 +1,30 @@
+package onspring
+
+import "net/http"
+
+// Option is a functional option for configuring a Client.
+type Option func(*Client)
+
+// WithHTTPClient sets a custom HTTP client for the Onspring client.
+// If not provided, the client will use http.DefaultClient.
+func WithHTTPClient(client *http.Client) Option {
+	return func(c *Client) {
+		c.httpClient = client
+	}
+}
+
+// WithBaseURL sets a custom base URL for the Onspring API.
+// This is useful for testing or when using a different Onspring environment.
+func WithBaseURL(url string) Option {
+	return func(c *Client) {
+		c.baseURL = url
+	}
+}
+
+// WithAPIVersion sets a custom API version for the Onspring client.
+// If not provided, the client will use the default API version.
+func WithAPIVersion(version string) Option {
+	return func(c *Client) {
+		c.apiVersion = version
+	}
+}