feat(routingv8): add billing tag to calculate matrix request

For more information see:
https://www.here.com/docs/bundle/cost-management-developer-guide/page/topics/tutorial-billing-tags.html

routingv8/calculatematrix.go

@@ -6,11 +6,17 @@ import (
 	"fmt"
 	"net/http"
 	"net/url"
+	"regexp"
 )
 
+const billingTagPattern = "^(?![-])[A-Za-z0-9_-]{4,16}(?<![_-])"
+
 func (c *CalculateMatrixRequest) QueryString() string {
 	values := make(url.Values)
 	values.Add("async", c.Async.String())
+	if c.BillingTag != "" {
+		values.Add("billingTag", c.BillingTag)
+	}
 	return values.Encode()
 }
 
@@ -27,6 +33,15 @@ func (s *MatrixService) CalculateMatrix(
 			err = fmt.Errorf("calculate matrix: %v", err)
 		}
 	}()
+	if req.BillingTag != "" {
+		matched, err := regexp.MatchString(billingTagPattern, req.BillingTag)
+		if err != nil {
+			return nil, err
+		}
+		if !matched {
+			return nil, fmt.Errorf("invalid billing tag")
+		}
+	}
 	u, err := s.URL.Parse("matrix")
 	if err != nil {
 		return nil, err

routingv8/request.go

@@ -42,6 +42,8 @@ type CalculateMatrixRequest struct {
 	// Async flag requires the Client to poll the calculation results and finally requesting to download
 	// the calculation results.
 	Async Async
+	// The billing tag provides a way to track your platform usage across components which track usage and costs.
+	BillingTag string
 	// Body to pass to request to Here Maps API
 	Body *CalculateMatrixBody
 }