Add a generic key loading helper function for xkey provider

Add a generic key loading helper function for xkey provider

- Load keys by specifying the opaque private key handle,
  public key, sign-op and free-op required for loading keys
  from Windows store and pkcs11.

- xkey_load_management_key is refactored to use the new function

- Also make xkey_digest non-static

Used in following commits to load CNG and pkcs11 keys

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20211214165928.30676-14-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23436.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>