Allow a requester to query for availability in the iroh-bytes sync protocol

[rklaehn]

Currently the only option a requester has is to download things from a provider. There is no way to ask a provider about available data. This makes it hard to deal with situations where we don't know if the provider has all data, and to divide a download to multiple providers.

We need a way for a requester to ask for

• available chunks for a blob
• available blobs and chunks for an entire collection

[flub]

What about available chunks for specific ranges in the blob/collection? Or is this likely to not be worth it and you may as well always simply request the available ranges of the entire thing?

And should there also be a shortcut to also fetching the available ranges? If you only have this new request to see what is available you'd have an extra RTT to fetch everything available. But you could have a request combining that with a fetch straight away. Possibly again worth seeing how far we get without?