Add maps to Echo Show skill

[zugaldia]

With https://github.com/mapbox/mapbox-assistant-example/commit/0b348d5f01cc72a197f79dc519ccf6f6563476df we added support for images in cards, which allows maps to be shown on Echo Show:

However, it looks like the standard card template doesn't let you to have the images to be full screen, or control the final size of the image. We need to research if there're other ways for maps to take a larger screen real state.

cc: @pveugen