Added support for changing geojson layer opacity

[giswqs]

Thanks to ipyleaflet v0.17.0, leafmap now supports changing GeoJSON layer visibility and opacity interactively.

@davidbrochart @martinRenou @jasongrout

import leafmap
m = leafmap.Map()
m.add_basemap("CartoDB.DarkMatter")
in_geojson = 'https://raw.githubusercontent.com/giswqs/geemap/master/examples/data/cable-geo.geojson'
callback = lambda feat: {"color": "#" + feat["properties"]["color"], "weight": 2}
m.add_geojson(in_geojson, layer_name="Cable lines", style_callback=callback, info_mode=None)
m