Add raster layer gallery

[martinRenou]

cc. @gjmooney

Add a raster layer gallery using xyzservices.

Common sense would want to create a server endpoint for accessing the gallery items, I'm not taking this approach otherwise it would not work in the case of jupyterlite.

Remaining for this PR:

• [ ] Should we really commit the images or include them at build time?
• [x] Missing attribution entry and some other raster layer parameters
• [ ] Make the layer gallery registry a plugin

[github-actions[bot]]

:point_left: Launch a Binder on branch _martinRenou/jupytergis/raster_layergallery

[davidbrochart]

A raster layer gallery sounds nice, but should it be implemented as a registry and populated by plugins?

[martinRenou]

A raster layer gallery sounds nice, but should it be implemented as a registry and populated by plugins?

Yeah definitely 👍🏽 Added it to the TODO list of the PR