Make Ed25519 consistent with X25519

Rename and change ED25519_keypair_from_seed to ED25519_public_from_private
to be consistent with X25519 API.

Modidy ED25519_sign to take separate public key argument instead of
requiring it to follow the private key.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3503)