feat: Improve optional arguments in show

[AlejandroFernandezLuces]

Overview

This PR fixes an unexpected behaviour in the show method, where optional keyword arguments were being passed only to plot in case users were passing plottable objects to the show method.

This caused that optional keyword arguments that needed to be passed to the underlying PyVista method show weren't reaching it. If you passed the plottable object and the optional keyword arguments destinated to the pyvista show method, an error would be thrown.

New behaviour

Now if you need to pass plottable objects directly to show, you can pass a dictionary through ``plotting_options` argument where you can add the PyVista plotting options for that plottable object.

Before

    pl = Plotter()
    sphere = pv.Sphere()

    # breaks
    pl.show(sphere, show_edges=True, cpos="xy")
    
    
    # Correct way
    pl.plot(sphere, show_edges=True)
    pl.show(cpos="xy")
    

Now

    pl = Plotter()
    sphere = pv.Sphere()
    pl.show(sphere, plotting_options={"show_edges": True}, cpos="xy")

Associated issues

ansys/pymechanical#1189

[SMoraisAnsys]

LGTM, I left a minor comment that you can discard if needed