libbpf: move libbpf_sha256() implementation into libbpf_utils.c

Move sha256 implementation out of already large and unwieldy libbpf.c
into libbpf_utils.c where we'll keep reusable helpers.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20251001171326.3883055-5-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>