Move attribute fields to a substructure

Move the "core attributes" to a substructure of psa_key_attribute_t.
The motivation is to be able to use the new structure
psa_core_key_attributes_t internally.
diff --git a/library/psa_crypto_slot_management.c b/library/psa_crypto_slot_management.c
index e63dcda..6add6b8 100644
--- a/library/psa_crypto_slot_management.c
+++ b/library/psa_crypto_slot_management.c
@@ -133,7 +133,7 @@
         goto exit;
     p_slot->lifetime = psa_get_key_lifetime( &attributes );
     p_slot->type = psa_get_key_type( &attributes );
-    p_slot->policy = attributes.policy;
+    p_slot->policy = attributes.core.policy;
 
 #if defined(MBEDTLS_PSA_CRYPTO_SE_C)
     if( psa_key_lifetime_is_external( p_slot->lifetime ) )