Feature request: enable custom selection of ppi for plots

As it stands now, the ppi parameter used to render all kinds of plots is globally defined here:
https://github.com/rstudio/py-shiny/blob/59bbe557dc10cdacca43be2895b5e8c627572b75/shiny/render/_render.py#L191-L194

This is later on passed to all subsequent calls until either matplotlib's savefig() or plotnine/PIL's save() methods.

Passing kwargs that end up being passed to those methods with the @render.plot decorator is allowed, but since the previously defined self._ppi is passed as an arg, adding the dpi kwarg with the decorator leads to a multiple values for argument error.

Could it be made so that the self._ppi attribute is updated at instantiation time in cased it is passed as a kwarg and thus allowing more control on figure resolution?

Start in shiny/render/_render.py around the global ppi definition and follow how @render.plot passes arguments through to matplotlib savefig() or plotnine/PIL save(). Confirm how a decorator-provided dpi or ppi should update the instance value without creating duplicate arguments, and add coverage showing that custom resolution reaches the plot-saving call.