Add Google Hybrid, Esri Satellite, and HERE Hybrid base layers

[svet-b]

@amotl just wanted to submit this following your comment on https://github.com/grafana/worldmap-panel/pull/210#issuecomment-516542791

First of all, we massively appreciate your work and improvements on this plugin. They're nothing short of transformational in terms of its usefulness.

This PR adds three additional base layers, with a focus on satellite imagery. Namely Google Maps Hybrid, Esri Satellite, and HERE Hybrid. The default is set to Google Hybrid.

Note that merging this PR as it stands will straight up set the base layer to Google Hybrid - which may not be what people expect to see. In that respect, one major shortcoming is that there is still no UI selector for which base layer to actually use. At the moment this can be set manually by editing worldmap_ctrl.ts.