feat: multi-provider fan-in

[b5]

download_with_opts currently allows multiple node addresses, but fetches in series. download_with_opts should be modified/expanded to support dividing the request up across multiple nodes, fanning the goodput responses in to a single response stream.

@arilotter is currently adding this on her own downstream out of urgent need. We should try to help, at least through code guidance, considering we've wanted this in iroh blobs for some time.

[rklaehn]

So you just want to do a fixed plan, not some dynamic thing?

[arilotter]

So you just want to do a fixed plan, not some dynamic thing?

being able to add a second node as a download source after the blob starts downloading from a first node would be really useful - a dynamic thing seems like it would be awesome long-term, but something fixed like "if you have 2 nodes, download half the file from each" would be way better than nothing