Declare multipart AEAD functions

Declare and document multipart AEAD functions.

This commit does not contain any implementation or tests.

include/psa/crypto_sizes.h

@@ -268,6 +268,27 @@
      (plaintext_length) + PSA_AEAD_TAG_LENGTH(alg) :              \
      0)
 
+/** The maximum size of the output of psa_aead_finish(), in bytes.
+ *
+ * If the size of the ciphertext buffer is at least this large, it is
+ * guaranteed that psa_aead_finish() will not fail due to an
+ * insufficient buffer size. Depending on the algorithm, the actual size of
+ * the ciphertext may be smaller.
+ *
+ * \param alg                 An AEAD algorithm
+ *                            (\c PSA_ALG_XXX value such that
+ *                            #PSA_ALG_IS_AEAD(alg) is true).
+ *
+ * \return                    The maximum trailing ciphertext size for the
+ *                            specified algorithm.
+ *                            If the AEAD algorithm is not recognized, return 0.
+ *                            An implementation may return either 0 or a
+ *                            correct size for an AEAD algorithm that it
+ *                            recognizes, but does not support.
+ */
+#define PSA_AEAD_FINISH_OUTPUT_SIZE(alg, plaintext_length)      \
+    ((size_t)0)
+
 /** The maximum size of the output of psa_aead_decrypt(), in bytes.
  *
  * If the size of the plaintext buffer is at least this large, it is