Prototypes for AEAD functions

This is still tentative.
diff --git a/include/psa/crypto_struct.h b/include/psa/crypto_struct.h
index ffa835e..9e70512 100644
--- a/include/psa/crypto_struct.h
+++ b/include/psa/crypto_struct.h
@@ -105,4 +105,17 @@
     } ctx;
 };
 
+struct psa_aead_operation_s
+{
+    psa_algorithm_t alg;
+    int key_set : 1;
+    int iv_set : 1;
+    int ad_set : 1;
+    uint8_t iv_size;
+    uint8_t block_size;
+    union
+    {
+    } ctx;
+};
+
 #endif /* PSA_CRYPTO_STRUCT_H */