Support sending DigestSign request to management client

Support sending DigestSign request to management client

To receive undigested message for signing, indicate support
for handling message digesting in the client using an argument
"digest" to --management-external-key.

For example, to announce pkcs1 padding and digesting support use:

--management-external-key pkcs1 pss digest

In PK_SIGN, the algorithm string will get data=message
in addition to other relevant options.

Note that it is not guaranteed that the client will be prompted
with undigested message. This is possible only when OpenSSL
calls our provider for DigestSign() as opposed to Sign(). In
practice, signature operation always appears to result in
a DigestSign() call through the provider interface.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20211214165928.30676-12-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23435.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>