MM-14441: restrict system admin config (#10477)

* tweak utils.Merge docs

* move merge_test to utils_test package for easier testing

* utils: support MergeConfig and StructFieldFilter

* constrain updating certain fields by the restricted system admin

config/utils.go

@@ -128,3 +128,15 @@ func FixInvalidLocales(cfg *model.Config) bool {
 
 	return changed
 }
+
+// Merge merges two configs together. The receiver's values are overwritten with the patch's
+// values except when the patch's values are nil.
+func Merge(cfg *model.Config, patch *model.Config, mergeConfig *utils.MergeConfig) (*model.Config, error) {
+	ret, err := utils.Merge(cfg, patch, mergeConfig)
+	if err != nil {
+		return nil, err
+	}
+
+	retCfg := ret.(model.Config)
+	return &retCfg, nil
+}