NetworkPkg/IScsiDxe: support SHA256 in CHAP

NetworkPkg/IScsiDxe: support SHA256 in CHAP

Insert a SHA256 CHAP_HASH structure at the start of "mChapHash".

Update ISCSI_CHAP_MAX_DIGEST_SIZE to SHA256_DIGEST_SIZE (32).

This enables the initiator and the target to negotiate SHA256 for CHAP, in
preference to MD5.

Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3355
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Message-Id: <20210629163337.14120-6-lersek@redhat.com>