MEDIUM: sample: Get chunks with a size dependent on input data when necessary

MEDIUM: sample: Get chunks with a size dependent on input data when necessary

The function used to duplicate a sample was update to support large buffers. In
addition several converters were also reviewed to deal with large buffers. For
instance, base64 encoding and decoding must use chunks of the same size than the
sample. For some of them, a retry is performed to enlarge the chunk is possible.

TODO: Review reg_sub, concat and add_item to get larger chunk if necessary