add contract structure from Merchant

src/include/taler_signatures.h

@@ -843,6 +843,25 @@ struct TALER_DepositTrackPS
 };
 
 
+/**
+ * The contract sent by the merchant to the wallet.
+ */
+struct TALER_ContractPS
+{
+  /**
+   * Purpose header for the signature over the contract with
+   * purpose #TALER_SIGNATURE_MERCHANT_CONTRACT.
+   */
+  struct GNUNET_CRYPTO_EccSignaturePurpose purpose;
+
+  /**
+   * Hash of the JSON contract in UTF-8 including 0-termination,
+   * using JSON_COMPACT | JSON_SORT_KEYS
+   */
+  struct GNUNET_HashCode h_contract;
+
+};
+
 
 
 GNUNET_NETWORK_STRUCT_END