utils: add `fragmented_temporary_buffer::allocate_to_fit`
utils: add `fragmented_temporary_buffer::allocate_to_fit`

Introduce `fragmented_temporary_buffer::allocate_to_fit` static
function returning an instance of the buffer of a specified size.

The allocated buffer fragments have a size of at most 128kb.
`bytes_ostream` has the same hard-coded limit, so just use the
same here.

This patch will be later needed for `raft::log_entry` raw data
serialization when writing to the underlying persistent storage.

Signed-off-by: Pavel Solodovnikov <2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla