Add `Powered by Apache OpenDAL` logo to Readme

[simonsan]

I would find it nice of us to show support in our readme and add their newly created logo (as they seem to come out of incubator phase): https://www.apache.org/logos/?#opendal to our readme of rustic_core / rustic_backend

Maybe adding a white background or something, as it seems hard to see on dark mode.

[Xuanwo]

Thanks a lot for your support! We have a dark version of logo, not sure if it will be better for you to use: https://github.com/apache/opendal/blob/main/website/static/img/logo_dark.svg