Add a function that returns the content of the ChunkedFile

[agazso]

The Bee API works in a way that the /chunks endpoints expects that the data to be uploaded alongside with the span. Downloading also gives you back the 8 bytes span + the actual payload data.

In the Book of Swarm this (the span + payload) is referred as content. It would be good to have a function called content on the ChunkedFile interface that would return this so that it's easier to work with the Bee API.