net: buf: Add net_buf_simple_clone

Provides a way to clone a net_buf_simple without altering the state of
the original buffer. The primary usage scenario is for manipulating a
previously allocated PDU inside a buffer without altering the length and
offset of the buffer.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>