image url about flutter_echarts

[entronad]

@lironiz

Because this lib abstract web assets to strings, you could not directly handle imge files.
The solution is to use Data URL , Echarts image property allows all legal URL forms, so just transform your image asset to a Base64 Data URL string just like we often do in CSS.
If you are not familiar with CSS Base64 Data URL, please search for docs like: https://stackoverflow.com/questions/17090571/is-there-a-way-to-set-background-image-as-a-base64-encoded-image