trailer: libify a couple of functions

trailer: libify a couple of functions

Move create_in_place_tempfile() and process_trailers() from
builtin/interpret-trailers.c into trailer.c and expose it via trailer.h.

This reverts most of ae0ec2e0e0b (trailer: move interpret_trailers()
to interpret-trailers.c, 2024-03-01) and lets other call sites reuse
the same trailer rewriting logic.

Signed-off-by: Li Chen <me@linux.beauty>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>