use istio client-go library instead of knative (#1661)

use istio client-go library instead of knative
bump kubernetes dependency version
change code coverage to codecov

vendor/github.com/go-openapi/spec/items.go

@@ -29,6 +29,7 @@ const (
 // SimpleSchema describe swagger simple schemas for parameters and headers
 type SimpleSchema struct {
 	Type             string      `json:"type,omitempty"`
+	Nullable         bool        `json:"nullable,omitempty"`
 	Format           string      `json:"format,omitempty"`
 	Items            *Items      `json:"items,omitempty"`
 	CollectionFormat string      `json:"collectionFormat,omitempty"`
@@ -91,6 +92,12 @@ func (i *Items) Typed(tpe, format string) *Items {
 	return i
 }
 
+// AsNullable flags this schema as nullable.
+func (i *Items) AsNullable() *Items {
+	i.Nullable = true
+	return i
+}
+
 // CollectionOf a fluent builder method for an array item
 func (i *Items) CollectionOf(items *Items, format string) *Items {
 	i.Type = jsonArray