Add base code to load a SLH_DSA public key.

This loads a SLH_DSA public key from data.
A simple SLH_DSA keymanager imports this key.

Initially this only has a parameter set for
SLH-DSA-SHA2-128s

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)

include/crypto/slh_dsa.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2024 The OpenSSL Project Authors. All Rights Reserved.
+ *
+ * Licensed under the Apache License 2.0 (the "License").  You may not use
+ * this file except in compliance with the License.  You can obtain a copy
+ * in the file LICENSE in the source distribution or at
+ * https://www.openssl.org/source/license.html
+ */
+
+/* Internal SLH_DSA functions for other submodules, not for application use */
+
+#ifndef OSSL_CRYPTO_SLH_DSA_H
+# define OSSL_CRYPTO_SLH_DSA_H
+
+# pragma once
+# include <openssl/e_os2.h>
+# include <openssl/types.h>
+# include "crypto/types.h"
+
+SLH_DSA_KEY *ossl_slh_dsa_key_new(OSSL_LIB_CTX *libctx, const char *alg);
+void ossl_slh_dsa_key_free(SLH_DSA_KEY *key);
+int ossl_slh_dsa_key_up_ref(SLH_DSA_KEY *key);
+int ossl_slh_dsa_key_equal(const SLH_DSA_KEY *key1, const SLH_DSA_KEY *key2,
+                           int selection);
+int ossl_slh_dsa_key_has(const SLH_DSA_KEY *key, int selection);
+int ossl_slh_dsa_key_fromdata(SLH_DSA_KEY *key, const OSSL_PARAM *params);
+
+#endif /* OSSL_CRYPTO_SLH_DSA_H */

include/crypto/types.h

@@ -28,6 +28,9 @@ typedef struct dsa_st DSA;
 # ifndef OPENSSL_NO_EC
 typedef struct ecx_key_st ECX_KEY;
 # endif
+# ifndef OPENSSL_NO_SLH_DSA
+typedef struct slh_dsa_key_st SLH_DSA_KEY;
+# endif
 
 typedef struct prov_skey_st PROV_SKEY;