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/usage_api.go

@@ -0,0 +1,61 @@
+package telecom
+
+import (
+	"context"
+	"fmt"
+	"time"
+)
+
+// UsageAPI handles usage-related API calls
+type UsageAPI struct {
+	client *HTTPClient
+}
+
+// NewUsageAPI creates a new UsageAPI
+func NewUsageAPI(client *HTTPClient) *UsageAPI {
+	return &UsageAPI{client: client}
+}
+
+// GetStats retrieves usage statistics for a subscriber
+func (u *UsageAPI) GetStats(ctx context.Context, subscriberID int64, startDate, endDate time.Time) (*UsageStats, error) {
+	params := map[string]string{
+		"start_date": startDate.Format(time.RFC3339),
+		"end_date":   endDate.Format(time.RFC3339),
+	}
+
+	var stats UsageStats
+	err := u.client.Get(ctx, fmt.Sprintf("/v1/subscribers/%d/usage", subscriberID), &stats, params)
+	return &stats, err
+}
+
+// ListEvents retrieves a list of usage events
+func (u *UsageAPI) ListEvents(ctx context.Context, subscriberID int64, usageType string, startDate, endDate time.Time, page, pageSize int32) (*UsageEventList, error) {
+	params := map[string]string{
+		"page":      fmt.Sprintf("%d", page),
+		"page_size": fmt.Sprintf("%d", pageSize),
+	}
+
+	if subscriberID > 0 {
+		params["subscriber_id"] = fmt.Sprintf("%d", subscriberID)
+	}
+	if usageType != "" {
+		params["usage_type"] = usageType
+	}
+	if !startDate.IsZero() {
+		params["start_date"] = startDate.Format(time.RFC3339)
+	}
+	if !endDate.IsZero() {
+		params["end_date"] = endDate.Format(time.RFC3339)
+	}
+
+	var list UsageEventList
+	err := u.client.Get(ctx, "/v1/usage/events", &list, params)
+	return &list, err
+}
+
+// GetRealtime retrieves real-time usage for a subscriber
+func (u *UsageAPI) GetRealtime(ctx context.Context, subscriberID int64) (*RealTimeUsage, error) {
+	var usage RealTimeUsage
+	err := u.client.Get(ctx, fmt.Sprintf("/v1/subscribers/%d/realtime", subscriberID), &usage)
+	return &usage, err
+}