Add ED25519 ASN.1 method

Make X25519 key method more flexible by removing hard coding of NID_X25519
OID. Since the parameters and key syntax between ED25519 and X25519 are
almost identical they can share a lot of common code.

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