Add a option to leave existing auto label unchanged on update even when label field is hidden

Currently the module has 2 options:
- generate a label every time entity is saved and hide label field.
- show label field and generate a label only when field is left empty.

It lacks a 3rd option:
- Hide the label field and generate a label only when current label is empty.

This way, label would be generated on first save but not modified on later updates, unless it was programmatically reset somehow before update.

In my case, I generate a custom label with a random part. But I don't want this random part to change each time entity is updated. This is not possible without this 3rd option.