Implement ED448 and ED25519 support in xkey_provider

Implement ED448 and ED25519 support in xkey_provider

OpenSSL's implementation of ED448 and ED25519 has a few idiosyncrasies.
Instead of belonging to the elliptic curve type or to a common Edwards
curve type, ED448 and ED25519 have each their own type.

Also, OpenSSL expects signatures using these curves to be done with the
EVP_DigestSign API instead of the EVP_Sign API but using md=NULL.

This has been tested using a "fake" external key that used a normal
software key instead of a hardware implementation but that makes no
difference from the perspective of xkey_provider/management interface.

Patch v2: remove name functions from ed448/ed25519, ensure md is NULL
          for ed448/ed25519 and handle NULL/none better in general.
Patch v3: do not pass NULL as string for the OSSL params.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: <20220516104807.2568937-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg24363.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>