Allow the ca application to use EdDSA

Using the ca application to sign certificates with EdDSA failed because it
is not possible to set the digest to "null". This adds the capability and
updates the documentation accordingly.

Fixes #6201

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6286)
3 files changed