SLH-DSA clean ups

SLH-DSA clean ups

- Make slh_dsa_sign() return the siglen when sig is NULL.
- Remove the ability in fromdata to generate the public key root
  given the private key and public key seed. This was messy and can
  be done by key generation instead.
- Add common EVP_PKEY gettablesto SLH_DSA keys
  (OSSL_PKEY_PARAM_BITS, OSSL_PKEY_PARAM_SECURITY_BITS, and
   OSSL_PKEY_PARAM_MAX_SIZE).
- Update tests based on the above changes.

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25882)