Better API for multipart uploads

[nikeee]

Currently, it's implemented with their functions mapped to methods of the S3 Client.

Maybe we can design a better API surface that is like S3File.

Maybe something like:

const upload = client.multipartUpload("key");
await upload.uploadPart(uploadId, partNumber, data);
const parts = await upload.parts();
const partsI = await upload.partsIterating();
await upload.complete();
await upload.abort();