Support for downloading basemaps from MapServers

[kbvernon]

Is your feature request related to a problem? Please describe.
No problem, just like making maps in R...

Describe the solution you'd like
Maybe a function like arc_basemap(). Its api and internals would be almost identical to arc_raster().

Describe alternatives you've considered
Only other way I know to do this is working directly with {httr2}.

Additional context
One potential complication is that you might want to let the user decide whether they want to read it back in with terra::rast() or, for example, png::readPNG().