[feature request]Add a ImageChain.Download() function

[PhotonSPK]

Describe features you've considered / 新功能描述

provide a function that could download a image in ImageChain as byte[] (etc..). So i don't need to handle it manually.

[TheSnowfield]

In the current architecture, we have to ref Bot instance from the ImageChain if we do that. This will bring some troubles.