API: add ddof=1 argument to expanding/rolling_cov()
I think `expanding/rolling_cov()` should support a `ddof` argument, the way `expanding/rolling_std/var()` do (though the latter is not documented; see #8064). So long as `ddof` defaults to `1`, it will be backwards compatible, so I don't see any downsides. This is very easy/straightforward.
