Enable signing using CNG through xkey provider

Enable signing using CNG through xkey provider

- Add xkey_cng_sign() as sign_op for the provider
  and load the key using xkey_generic_load.

- Enable/Disable old code when provider is available or not.

- xkey_digest is made non-static for use in cryptoapi.c

One function cng_padding_type() is moved down to reduce number
of ifdef's.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20211214165928.30676-16-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23444.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>