utils: reusable_buffer: support fragmented_temporary_buffer
utils: reusable_buffer: support fragmented_temporary_buffer

reusable_buffer already supports bytes_ostream which is often used for
handling data sent from Scylla. This patch adds support for
fragmented_temporary_buffer which is going to be mainly used for data
received by Scylla.

duarten/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla