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/brands_test.go

@@ -1,6 +1,7 @@
 package ozon
 
 import (
+	"context"
 	"net/http"
 	"testing"
 
@@ -52,7 +53,8 @@ func TestListCertifiedBrands(t *testing.T) {
 	for _, test := range tests {
 		c := NewMockClient(core.NewMockHttpHandler(test.statusCode, test.response, test.headers))
 
-		resp, err := c.Brands().List(test.params)
+		ctx, _ := context.WithTimeout(context.Background(), testTimeout)
+		resp, err := c.Brands().List(ctx, test.params)
 		if err != nil {
 			t.Error(err)
 		}