Add admin get/set config keys API (#6113)

This PR adds two new admin APIs in Minio server and madmin package:
- GetConfigKeys(keys []string) ([]byte, error)
- SetConfigKeys(params map[string]string) (err error)

A key is a path in Minio configuration file, (e.g. notify.webhook.1)

The user will always send a string value when setting it in the config file,
the API will know how to convert the value to the appropriate type. The user
is also able to set a raw json.

Before setting a new config, Minio will validate all fields and try to connect
to notification targets if available.

cmd/config-migrate.go

@@ -2428,7 +2428,7 @@ func migrateV27ToV28MinioSys(objAPI ObjectLayer) error {
 
 	srvConfig.Version = "28"
 	srvConfig.KMS = crypto.KMSConfig{}
-	if err = saveServerConfig(objAPI, srvConfig); err != nil {
+	if err = saveServerConfig(context.Background(), objAPI, srvConfig); err != nil {
 		return fmt.Errorf("Failed to migrate config from ‘27’ to ‘28’. %v", err)
 	}