[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>

services/mailservice/mail_test.go

@@ -129,7 +129,7 @@ func TestSendMailUsingConfig(t *testing.T) {
 
 	fsInner, err := config.NewFileStore("config.json", false)
 	require.Nil(t, err)
-	fs, err := config.NewStoreFromBacking(fsInner)
+	fs, err := config.NewStoreFromBacking(fsInner, nil)
 	require.Nil(t, err)
 
 	cfg := fs.Get()
@@ -170,7 +170,7 @@ func TestSendMailWithEmbeddedFilesUsingConfig(t *testing.T) {
 
 	fsInner, err := config.NewFileStore("config.json", false)
 	require.Nil(t, err)
-	fs, err := config.NewStoreFromBacking(fsInner)
+	fs, err := config.NewStoreFromBacking(fsInner, nil)
 	require.Nil(t, err)
 
 	cfg := fs.Get()
@@ -217,7 +217,7 @@ func TestSendMailUsingConfigAdvanced(t *testing.T) {
 
 	fsInner, err := config.NewFileStore("config.json", false)
 	require.Nil(t, err)
-	fs, err := config.NewStoreFromBacking(fsInner)
+	fs, err := config.NewStoreFromBacking(fsInner, nil)
 	require.Nil(t, err)
 
 	cfg := fs.Get()