Add domain+signature to post transaction callback

umad-07-post-tx-hooks.md

@@ -21,7 +21,13 @@ each VASP to the other is called a "post-transaction hook". It is a POST request
 POST <utxoCallback>
 
 {
-  "utxos": { "utxo": string, "amountMsats": number }[]
+  "utxos": { "utxo": string, "amountMsats": number }[],
+  // Domain name of the VASP. Used when validating the signature.
+  "vaspDomain": string,
+  // The VASP's signature over sha256_hash(signatureNonce + signatureTimestamp),
+  "signature": string,
+  "signatureNonce": string,
+  "signatureTimestamp": number, // in seconds since epoch
 }
 ```