Add support for SLHDSA

[hsmguy]

Description

PR to add SLHDSA to the provider

new files:
keymgmt/slhdsa.c
sig/slhdsa.c

Code was mostly ported from the MLDSA implementation :-) so it looks familiar to you :-)
--took out MU as not present in SLHDSA
--left encoding, deterministic, and context
note: prehash still not implemented just like the mldsa code

Already did some testing:
--code compiles good
--provider initializes ok

(some testing done against an Entrust n5c rack mount hsm already):
--openssl genpkey slhdsa works (keymgmt/slhdsa.c new!)
--openssl export slhdsa key works, displays key ok
--(sign update) not working on entrust ncipher provider yet (only single part signing C_Sign not C_SignUpdate but the debug log got to that :-)

issue with req x509 cert routines:
--still need to find why openssl req failing, i think theres a bug in the ported code somewhere ooops! in objects.c the pubkey is 0 hmmmm think i screwed something up here. this is my only bug im tracking right now....
(trace output as follows)

(any helpers to find this bug? feel free to help!!!)

[root@rhel93sw139opssl352 nfast]# openssl req -key "pkcs11:token=accelerator;pin-value=1234;object=slhdsa22" -new -out req.csr

[2026-07-18 13:54:01.758] [../src/interface.gen.c:187] p11prov_GetSlotInfo(): Calling C_GetSlotInfo
[2026-07-18 13:54:01.759] [../src/interface.gen.c:187] p11prov_GetSlotInfo(): Calling C_GetSlotInfo

[2026-07-18 13:54:01.765] [../src/store.c:625] p11prov_store_close(): store close (0x5557e6489ef0)
[2026-07-18 13:54:01.765] [../src/store.c:35] p11prov_store_ctx_free(): store ctx free (0x5557e6489ef0)
[2026-07-18 13:54:01.765] [../src/obj/object.c:283] p11prov_obj_free(): Free Object: 0x5557e64dea80 (handle:1120)
[2026-07-18 13:54:01.765] [../src/obj/object.c:290] p11prov_obj_free(): object free: reference held
You are about to be asked to enter information that will be incorporated

[2026-07-18 13:54:06.370] [../src/encoder.c:328] p11prov_common_encoder_spki_der_encode(): DER Encoding SLH-DSA-SHA2-256s SubjectPublicKeyInfo
[2026-07-18 13:54:06.370] [../src/encoder.c:357] p11prov_common_encoder_spki_der_encode(): no pubkey
Error making certificate request
40375EAF607F0000:error:04000067:object identifier routines:OBJ_txt2obj:unknown object name:crypto/objects/obj_dat.c:435:
40375EAF607F0000:error:04000067:object identifier routines:OBJ_txt2obj:unknown object name:crypto/objects/obj_dat.c:435:
40375EAF607F0000:error:0580006F:x509 certificate routines:X509_PUBKEY_set:unsupported algorithm:crypto/x509/x_pubkey.c:367:
[2026-07-18 13:54:06.370] [../src/kmgmt/common.c:374] p11prov_kmgmt_free(): Free key 0x5557e64dea80 (type 75)
[2026-07-18 13:54:06.370] [../src/obj/object.c:283] p11prov_obj_free(): Free Object: 0x5557e64dea80 (handle:1120)
[2026-07-18 13:54:06.370] [../src/obj/pool.c:186] obj_rm_from_pool(): Object to be removed from pool (idx=0, obj=0x5557e64dea80)
[2026-07-18 13:54:06.370] [../src/obj/object.c:283] p11prov_obj_free(): Free Object: (nil) (handle:0)

code is 99.9999 % done !

(1) 1 bug to find above in openssl req objects, the pubkey is running away hmmm not sure why yet need to spend some time on this.... prolly made a mistake somewhere when porting over the code hmmm

(2) should SLHDSA be made single part signing? or leave as multipart (pkcs11 3.2) signing?

Checklist

• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• This feature/change has adequate documentation added
• Code conform to coding style that today cannot yet be enforced via the check style test
• Commits have short titles and sensible commit messages

[hsmguy]

Hi @simo5 code is mostly done 99.9999 % :-) and its YOUR code from mldsa too, slightly modded.
lots of work done x 12 params :-)

could use some help on the PR to take it further

thank you