Ideas for reintroducing `fill_value` to Lag primitive

After discussing https://github.com/alteryx/featuretools/pull/2252 we decided to remove `fill_value` from `Lag` since it may be a problem with multiple types of columns in DFS. If there is a need to bring this back we proposed a few ideas:

- introduce `fill_value` but induce another parameter that allows the user to overwrite the input_types. That way DFS will only apply lag to one type and `fill_value` can be applied safely.
- make `fill_value` default to an empty dictionary where the user can overwrite that and specify different fill values for different types of columns. eg `{"categorical": "foo", "numeric":0}`