API: engine kw to .plot to enable selectable backends
We have had some conversations in the past regarding an `.plot(....., engine=)` kw. in #8018 
This would allow pandas to redirect the plotting to a user selectable back-end, keeping `matplotlib` as the default.

see [chartpy here](https://github.com/cuemacro/chartpy), for a way to selectively enable `matplotlib`, `bokeh` and `plotly`.

and generically via [altair](https://github.com/ellisonbg/altair).

Missing from here is when to re-direct to `seaborn`.

So this issue is for discussion:

1) should we do this
2) implementation method and minimalist dependencies to actually do this (ideally pandas would add _NO_ dependences itself, just import for a particular engine, raising if its not available).
3) maybe should spin off much of the current pandas code into a separate repo (`pandas-plot`)?
and this actually should be the default (rather than `matplotlib`), which is of course the dependency. This might allow simply removing the vast majority of the custom plotting code.
