Improve the strict configuration validation (#3809)

pkg/config/v1/proxy.go

@@ -15,6 +15,7 @@
 package v1
 
 import (
+	"bytes"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -177,7 +178,11 @@ func (c *TypedProxyConfig) UnmarshalJSON(b []byte) error {
 	if configurer == nil {
 		return fmt.Errorf("unknown proxy type: %s", typeStruct.Type)
 	}
-	if err := json.Unmarshal(b, configurer); err != nil {
+	decoder := json.NewDecoder(bytes.NewBuffer(b))
+	if DisallowUnknownFields {
+		decoder.DisallowUnknownFields()
+	}
+	if err := decoder.Decode(configurer); err != nil {
 		return err
 	}
 	c.ProxyConfigurer = configurer