vtls: move sha256sum into the Curl_ssl struct

The SHA-256 checksumming is also an SSL backend-specific function.
Let's include it in the struct declaring the functionality of SSL
backends.

In contrast to MD5, there is no fall-back code. To indicate this, the
respective entries are NULL for those backends that offer no support for
SHA-256 checksumming.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>