IMPACT: Plot shakemaps, ruptures and assets on top of basemaps obtained from external tiles services

[ptormene]

Based on https://github.com/gem/oq-engine/pull/10157

It introduces a dependency from the package contextily (if we decide to merge this PR, we have to add that package to our weelhouse)

Some examples using CartoDB.Positron (we may choose among lots of others):