[Feat] jupyter: Support custom CRS with Proj4Leaflet

[ninsbl]

Is your feature request related to a problem? Please describe.
User data is regularly in other CRS than WGS84 or WebMercator. It would be great if grass.jupyter integration could handle custom CRS to avoid reprojection. I remember there was a discussion earlier, but it seams leaflet now has better supprt for other CRS with the proj4leaflet plugin, see example here:
https://github.com/jupyter-widgets/ipyleaflet/blob/master/examples/CustomProjections.ipynb

Describe the solution you'd like
Maps in jupyter should be able to set the CRS according to the location / Project instead of allways reprojecting in order to avoid costly computations (however, I did not check (or clearly understand) what proj4leaflet actually does and if it really avoids reprojecting)...
It would add to dependencies though...

[petrasovaa]

I agree this should be explored. But the limitation will be the basemaps that are in pseudomercator only...