feat(auth): create a zero-OID for Subject Public Key

Created an explicit zero-OID which can be used for Subject
Public Key that do not have their own key identifier.

With this, all keys (including the subject public key) have
a proper key OID string so we don't need to make a special
case of null pointers when it comes to handling key OIDs.

Change-Id: Ice6923951699b6e253d7fd87e4c1b912470e0391
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/include/tools_share/dualroot_oid.h b/include/tools_share/dualroot_oid.h
index 3e88a6d..76fffba 100644
--- a/include/tools_share/dualroot_oid.h
+++ b/include/tools_share/dualroot_oid.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020, Arm Limited. All rights reserved.
+ * Copyright (c) 2020-2023, Arm Limited. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -9,6 +9,7 @@
 
 /* Reuse the Object IDs defined by TBBR for certificate extensions. */
 #include "tbbr_oid.h"
+#include "zero_oid.h"
 
 /*
  * Platform root-of-trust public key.