step_lag should have an order_by argument

[alexpghayes]

Currently it requires that the input tibble is appropriately sorted, but we could just pass a column to sort by to dplyr::lag in the implementation. Probably can get around to this at some point, but it's a low priority atm.

[EmilHvitfeldt]

I'm working on a package that handles timed data in recipes. So I think this type of feature would be better served as step_time_lag() that will be added to that package.

I will keep this issue open while I wait for the package to materialize and visibility