Consider migrating to object_store for all access

[nyurik]

It seems the object_store crate can handle a lot of the use cases for us - reading local and remote storage - all major cloud providers plus all regular HTTP, plus supports cache invalidation (including etag). We may want to just migrate to this instead of having our own backend support?

[lseelenbinder]

That looks like a really fascinating approach @nyurik! I'm not opposed at all. We can definitely test it immediately as another backend.