Expose message fragmentation

[skreborn]

Currently, Zenoh performs internal fragmentation for large messages. While this allows it to transfer the message reliably, it provides no way for the developer to access the progress of this process.

To circumvent this, one has to manually fragment messages, which introduces complexity as well as duplication of work and reduction of performance.

This would be most beneficial when transferring large binary blobs, like files, to keep track of the transfer process and provide accurate predictions about their expected delivery time.

One possibility would be to provide an interface that streams fragments in and out of publishers and subscribers respectively, which would have the added benefit of not necessarily having to store the entire blob in memory on either side of the transfer.