Add option to add background basemap to plots

[adehecq]

Is your feature request related to a problem? Please describe.
It would be great when making a plot to easily have a basemap in the background. I think it would be easy to do with contextily.

Describe the solution you'd like
Something like:

rst = gu.Raster("my_raster.tif")
rst.plot(basemap="OpenStreetMap")
plt.show()

Describe alternatives you've considered
No alternative considered.

Additional context
None

[rhugonnet]

Would be nice indeed. Here I refer to the plotting expertise of @friedrichknuth.
My guess is that, once we have the Xarray accessor, we will have access to plotting tools in the Xarray landscape that likely already support this?