Add support getting raw private/public keys

Only applies to algorithms that support it. Both raw private and public
keys can be obtained for X25519, Ed25519, X448, Ed448. Raw private keys
only can be obtained for HMAC, Poly1305 and SipHash

Fixes #6259

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6394)