use web assets to load dynamic image

[foxzool]

https://bevy.org/news/bevy-0-17/#web-assets

free api
https://api.unsplash.com/photos/random?client_id=yJSlkpXWNGkhGV2hYvtcKtMpf9TriC_IahE-EMJYEoc

[foxzool]

json can decode by https://github.com/foxzool/bevy_http_client