Ideas for reintroducing `fill_value` to Lag primitive

[ozzieD]

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}