Refactor OpenSSL 'EdDSA' EVP_SIGNATURE to allow use with EVP_PKEY functions

Add EVP_PKEY_{sign,verify}_message support for our Ed25519 and Ed448
implementations, including ph and ctx variants.

Tests are added with test_evp stanzas.

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)