pkg(7): use libmd for sha256 instead of openssl

OpenSSL 3.0 has deprecated the sha256 api, let's use libmd which has the
same API instead.

In order to avoid the collision in definitions (sha256.h cannot be
included in the same file as a file where openssl headers has been
included) let's move the sha256 related code in its own file

PR: 270023
Reported by: ngie