[MM-32234] Fix config.json getting reset (#16805)

* Add support for read-only config stores

* Allow read/write config store for plugin commands that require it

api4/apitestlib.go

@@ -96,7 +96,7 @@ func setupTestHelper(dbStore store.Store, searchEngine *searchengine.Broker, ent
 	}
 	memoryStore.Set(memoryConfig)
 
-	configStore, err := config.NewStoreFromBacking(memoryStore, nil)
+	configStore, err := config.NewStoreFromBacking(memoryStore, nil, false)
 	if err != nil {
 		panic(err)
 	}

api4/config_test.go

@@ -691,11 +691,11 @@ func TestMigrateConfig(t *testing.T) {
 	})
 
 	th.TestForSystemAdminAndLocal(t, func(t *testing.T, client *model.Client4) {
-		f, err := config.NewStore("from.json", false, nil)
+		f, err := config.NewStore("from.json", false, false, nil)
 		require.NoError(t, err)
 		defer f.RemoveFile("from.json")
 
-		_, err = config.NewStore("to.json", false, nil)
+		_, err = config.NewStore("to.json", false, false, nil)
 		require.NoError(t, err)
 		defer f.RemoveFile("to.json")