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/export_test.go

@@ -26,8 +26,3 @@ func InitializeConfigurationsTable(db *sqlx.DB) error {
 func ResolveConfigFilePath(path string) (string, error) {
 	return resolveConfigFilePath(path)
 }
-
-// GetCommonStore exposes the internal commonStore to test only.
-func (ms *memoryStore) MergeConfig(patch *model.Config) (*model.Config, error) {
-	return ms.mergeConfig(patch)
-}