New function: color legend and color proportion in folium.heatmap

Describe the solution you'd like
without change the default color in current folium heatmap.
we want to see a color legend can be show on map and different color proportion can be showed next to the color legend.

Describe alternatives you've considered
add a color legend can show the correspondence between clor and value
add a color proportion can tell user the percentage of different color, it is very convenient for making statistics.

Additional context
a example link can be seen here:
https://en.wikipedia.org/wiki/Heat_map#/media/File:LakeEffectSnowBuffalo101206.gif

Implementation
yes, I have used it in my example and I will share it with my friends.

首先查看 folium.heatmap 和链接的热力图示例，在不更改默认着色的情况下定义图例和颜色比例要求。确认所请求的可视化应如何显示在地图上；当地图在图例旁显示颜色与值的对应关系及比例时，即表示完成。