[MM-30436] Add support for configuration custom defaults (#16265)

* [MM-30436] Add support for configuration custom defaults

* Addressing review comments

* Addressing PR comments

* Soft fail if the configuration defaults cannot be read

* Directly print error in log message

* Fix linter

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

cmd/mattermost/commands/plugin_test.go

@@ -37,7 +37,7 @@ func TestPlugin(t *testing.T) {
 
 	fs, err := config.NewFileStore(th.ConfigPath(), false)
 	require.Nil(t, err)
-	cfsStore, err := config.NewStoreFromBacking(fs)
+	cfsStore, err := config.NewStoreFromBacking(fs, nil)
 	require.Nil(t, err)
 	require.NotNil(t, cfsStore.Get().PluginSettings.PluginStates["testplugin"])
 	assert.True(t, cfsStore.Get().PluginSettings.PluginStates["testplugin"].Enable)
@@ -47,7 +47,7 @@ func TestPlugin(t *testing.T) {
 	assert.Contains(t, output, "Disabled plugin: testplugin")
 	fs, err = config.NewFileStore(th.ConfigPath(), false)
 	require.Nil(t, err)
-	cfsStore, err = config.NewStoreFromBacking(fs)
+	cfsStore, err = config.NewStoreFromBacking(fs, nil)
 	require.Nil(t, err)
 	require.NotNil(t, cfsStore.Get().PluginSettings.PluginStates["testplugin"])
 	assert.False(t, cfsStore.Get().PluginSettings.PluginStates["testplugin"].Enable)