Add scalebar support

[hoxbro]

Fixes #5948

I'm not entirely sure what plot classes need to have a scalebar option: all of them or a selective few. For now, I have added it to RGB to start with.

Also, I am not sure how many of the Scalebar options we should have available.

import holoviews as hv
hv.extension("bokeh")

hv.RGB.load_image("5948_pollen.png").opts(scalebar=True, scale_opts={"background_fill_alpha": 1})

image_download