Basic interface for key policies

Get/set the policy of a key slot.

Opaque structure for key policies and field access functions.
diff --git a/include/psa/crypto_struct.h b/include/psa/crypto_struct.h
index 9e70512..c0a6738 100644
--- a/include/psa/crypto_struct.h
+++ b/include/psa/crypto_struct.h
@@ -118,4 +118,10 @@
     } ctx;
 };
 
+struct psa_key_policy_s
+{
+    psa_key_usage_t usage;
+    psa_algorithm_t alg;
+};
+
 #endif /* PSA_CRYPTO_STRUCT_H */