fix(ipvanish): updater user agent to work again

internal/updater/unzip/fetch.go

@@ -14,7 +14,11 @@ func (u *Unzipper) FetchAndExtract(ctx context.Context, url string) (
 	if err != nil {
 		return nil, err
 	}
-	request.Header.Set("User-Agent", "gluetun")
+	if userAgent, ok := ctx.Value(userAgentContextKey).(string); ok && userAgent != "" {
+		request.Header.Set("User-Agent", userAgent)
+	} else {
+		request.Header.Set("User-Agent", "gluetun")
+	}
 
 	response, err := u.client.Do(request)
 	if err != nil {

internal/updater/unzip/unzip.go

@@ -1,6 +1,7 @@
 package unzip
 
 import (
+	"context"
 	"net/http"
 )
 
@@ -8,6 +9,16 @@ type Unzipper struct {
 	client *http.Client
 }
 
+type contextKey int
+
+const (
+	userAgentContextKey contextKey = iota
+)
+
+func WithUserAgent(ctx context.Context, userAgent string) context.Context {
+	return context.WithValue(ctx, userAgentContextKey, userAgent)
+}
+
 func New(client *http.Client) *Unzipper {
 	return &Unzipper{
 		client: client,