Add ED25519 and ED448 support for EVP_PKEY_{sign,verify}_init_ex2()

In this mode, only the ph instances are supported, and must be set
explicitly through a parameter.  The caller is assumed to pass a
prehash to EVP_PKEY_{sign,verify}().

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24975)