MM-11931 Add support for AllowedUntrustedInternalConnections to be comma-separated (#11614)

* Add support for AllowedUntrustedInternalConnections to be comma-separated

* Add comprehensive test cases for fields splitting function

api4/openGraph_test.go

@@ -30,7 +30,7 @@ func TestGetOpenGraphMetadata(t *testing.T) {
 	}()
 	th.App.UpdateConfig(func(cfg *model.Config) { *cfg.ServiceSettings.EnableLinkPreviews = true })
 	th.App.UpdateConfig(func(cfg *model.Config) {
-		*cfg.ServiceSettings.AllowedUntrustedInternalConnections = "localhost 127.0.0.1"
+		*cfg.ServiceSettings.AllowedUntrustedInternalConnections = "localhost,127.0.0.1"
 	})
 
 	ogDataCacheMissCount := 0