Resumable uploads under application control

We would like the application to start uploads and continue them after the application restarts. The API should support something like:

```rust
// The application can checkpoint `id` in a database or other persistent storage, to resume after a restart.
let id: String = client.start_upload("bucket", "object").await?;

// Requires the payload to implement `Seek`. Unlike most uploads this does not finalize the upload if the stream is closed.
let payload = tokio::fs::File::open("my-file.txt").await?;
let status = client.continue_upload("bucket", "object", id, payload).send().await?;

// Object is only finalized explicitly. The final payload *could* be empty.
let empty_payload = "";
let object = client.finalize_upload("bucket", "object", id, empty_payload).send().await?;
```