SLH-DSA Update private key getter to also return public components.

SLH-DSA Update private key getter to also return public components.

The private key is defined in FIPS 205 as containing the public key,
so we return this also. This also matches what happens in fromdata.

Updated Documentation for SLH_DSA.

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26839)