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/common.go

@@ -4,7 +4,6 @@
 package config
 
 import (
-	"github.com/mattermost/mattermost-server/utils"
 	"io"
 	"sync"
 
@@ -141,14 +140,3 @@ func (cs *commonStore) validate(cfg *model.Config) error {
 
 	return nil
 }
-
-// mergeConfig merges two configs together. The receiver's values are overwritten with the patch's
-// values except when the patch's values are nil.
-func (cs *commonStore) mergeConfig(patch *model.Config) (*model.Config, error) {
-	ret, err := utils.Merge(cs.config, patch)
-	if err != nil {
-		return nil, err
-	}
-	retC := ret.(model.Config)
-	return &retC, nil
-}