Replace AWS SDK with object_store crate

[blitz]

The AWS SDK pulls in a gigantic dependency tree that pulls in outdated crates. As we only use S3, we can replace it with object_store.

[winterqt]

Happy to work on this, if that's okay with you.

[blitz]

@winterqt Yes, please!

[winterqt]

I'll probably migrate away from async_trait for server first, but sounds good!