增加sm2 public key parse demo (#1426)

* 添加公钥解析demo

demos/src/demo_sm2_public_key_parse.c

@@ -0,0 +1,32 @@
+/*
+ *  Copyright 2014-2022 The GmSSL Project. All Rights Reserved.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the License); you may
+ *  not use this file except in compliance with the License.
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ */
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <gmssl/mem.h>
+#include <gmssl/sm2.h>
+
+
+int main(void)
+{
+	SM2_KEY sm2_key;
+
+	printf("Read SM2 public key file (PEM) from stdin ...\n");
+	
+	if (sm2_public_key_info_from_pem(&sm2_key, stdin) != 1) {
+		fprintf(stderr, "error\n");
+		return 1;
+	}
+	
+	sm2_public_key_print(stdout, 0, 0, "SM2PublicKey", &sm2_key);
+
+	gmssl_secure_clear(&sm2_key, sizeof(sm2_key));
+	return 0;
+}