Support Tokio's AsyncSeek trait for downloaded blobs.

[maboesanman]

Currently Tokio's AsyncRead api is supported via the Reader struct
https://docs.rs/iroh-blobs/latest/iroh_blobs/rpc/client/blobs/struct.Reader.html

It would be useful for things like zip archive based blobs to be able to seek as well.

Possibly AsyncBufRead as well.

If there is a performance penalty, there could be different methods for returning AsyncRead and AsyncRead + AsyncSeek

[rklaehn]

Ah, now I get what you want. You just want a way to get a handle to the item downloaded by the store, that you can use either in sync land (Read/Seek) and in tokio flavoured async land (AsyncRead / AsyncSeek).

Yes, that is quite reasonable, and something we should definitely support. Unfortunately there are a lot of things in flux right now, so if we just hack it in we might lose a lot of work.

[rklaehn]

Made an issue for this: #60

Let's have the discussion in there.