feat(storage)!: retry loop for downloads

[chingor13]

Implements retry loop for download

The metadata is returned on the initial request so we want this part of the retry loop. Therefore, we change when the user calls await.

Metadata reading:

let download = storageClient.readObject(from: "my-bucket", object: "large-file.bin")
let metadata = try await download.metadata
print("Size: \(metadata.size) bytes, Generation: \(metadata.generation)")

Data reading:

let download = storageClient.readObject(from: "my-bucket", object: "large-file.bin")
for try await chunk in download.body {
  processChunk(chunk)
}

Since the metadata fetching and data stream are in the same request, we use a coordinator to manage the RPC call.

[coryan]

Nothing blocking, though maybe the behavior when the service returns more data than expected should be to return an error?