SLH_DSA: Make apps.c do_X509_REQ_verify() call work correctly.

SLH_DSA: Make apps.c do_X509_REQ_verify() call work correctly.

- Added sigid_algs for SLH_DSA such that OBJ_find_sigid_algs() works.
- OBJ_sn2nid() was also being called, so the SN form of SLH_DSA
  algorithms needed to be added to the provider dispatch tables.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26625)