Add EVP_DigestSign and EVP_DigesVerify

Add "single part" digest sign and verify functions. These sign and verify
a message in one function. This simplifies some operations and it will later
be used as the API for algorithms which do not support the update/final
mechanism (e.g. PureEdDSA).

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3409)