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.

client_export_test.go

@@ -0,0 +1,19 @@
+package onspring
+
+import "net/http"
+
+func (c *Client) HTTPClient() *http.Client {
+	return c.httpClient
+}
+
+func (c *Client) BaseURL() string {
+	return c.baseURL
+}
+
+func (c *Client) APIKey() string {
+	return c.apiKey
+}
+
+func (c *Client) APIVersion() string {
+	return c.apiVersion
+}