Configure HttpClient, added context parameter to all methods (#37)

Context is needed to limit time of execution of a method. Previously, context was passed to structure and it was stored inside structure which is a bad practice. Now we need to pass context to function, which is a best practice

ozon/common.go

@@ -1,5 +1,11 @@
 package ozon
 
+import "time"
+
+const (
+	testTimeout = 5 * time.Second
+)
+
 type Order string
 
 const (