Add support for non-projection EPSG codes

[jbmuir]

Description

It would be helpful for high precision work to be able to non-projection based coordinate systems (in my case, GDA94/GDA2020) which crs.epsg does not currently support as per https://scitools.org.uk/cartopy/docs/latest/reference/generated/cartopy.crs.epsg.html

e.g. having the following work.

import cartopy.crs as ccrs

src_coords_system = ccrs.epsg(7844) # GDA2020

[greglucas]

Seems like a reasonable request. I think it would mean creating a new class that subclasses CRS instead of Projection and then piping that through the function to return the proper class: https://github.com/SciTools/cartopy/blob/main/lib/cartopy/_epsg.py