API: Add Calendar Day Frequency ("CD")
This is a proposal to replace #20633.

`offsets.Day` (`'D'`) is documented to represent calendar day; however, `Day` arithmetic and usage of the `'D'` offset alias (e.g. `df.resample('D')`, `timedelta_range(..., freq='D')`, etc.) currently respects absolute time (i.e. `Day` acts like `Timedelta(days=1)`). This usage is ingrained pretty deeply in a lot of methods and operations and is fairly difficult to walk back to respect the notion of calendar day as stated in the docs.

Instead, I propose to keep `Day` as is (a timedelta-like frequency like `Hour` (`'H'`)) and add a new frequency `CalendarDay`, `'CD'`. It would act very similarly to `DateOffset(day=1)` but act like a frequency. Thoughts?
