docs: Add Go SDK README with comprehensive API documentation and implement modular architecture with authentication provider

- Add README.md with installation instructions, architecture overview, and API examples for all SDK modules
- Add AuthProvider class with API key and JWT token support (GenerateJWTToken, ValidateJWTToken, GetHeaders, ClearTokenCache methods)
- Add HTTPClient base class with retry logic, error handling, and timeout management
- Add GraphQLAPI, PaymentsAPI, RatingPlansAPI

sdk/go/system_api.go

@@ -0,0 +1,29 @@
+package telecom
+
+import (
+	"context"
+)
+
+// SystemAPI handles system-related API calls
+type SystemAPI struct {
+	client *HTTPClient
+}
+
+// NewSystemAPI creates a new SystemAPI
+func NewSystemAPI(client *HTTPClient) *SystemAPI {
+	return &SystemAPI{client: client}
+}
+
+// GetStats retrieves system statistics
+func (s *SystemAPI) GetStats(ctx context.Context) (*SystemStats, error) {
+	var stats SystemStats
+	err := s.client.Get(ctx, "/v1/system/stats", &stats)
+	return &stats, err
+}
+
+// GetHealth retrieves system health status
+func (s *SystemAPI) GetHealth(ctx context.Context) (*HealthStatus, error) {
+	var health HealthStatus
+	err := s.client.Get(ctx, "/v1/health", &health)
+	return &health, err
+}