Add a function for overriding the receiver domain in the lnurlp.

This is useful for cases where the receiver domain is not the same as the incoming request Host, for example when the
request is being proxied to another internal service.

uma/test/uma_test.go

@@ -117,6 +117,20 @@ func TestSignAndVerifyLnurlpRequest(t *testing.T) {
 	require.NoError(t, err)
 }
 
+func TestSignAndVerifyLnurlpRequestReplacingDomain(t *testing.T) {
+	privateKey, err := secp256k1.GeneratePrivateKey()
+	require.NoError(t, err)
+	queryUrl, err := uma.GetSignedLnurlpRequestUrl(privateKey.Serialize(), "[email protected]", "vasp1.com", true, nil)
+	require.NoError(t, err)
+	queryUrl.Host = "vasp2.com"
+	query, err := uma.ParseLnurlpRequestWithReceiverDomain(*queryUrl, "vasp3.com")
+	require.NoError(t, err)
+	require.Equal(t, *query.UmaVersion, uma.UmaProtocolVersion)
+	require.Equal(t, query.ReceiverAddress, "[email protected]")
+	err = uma.VerifyUmaLnurlpQuerySignature(*query.AsUmaRequest(), getPubKeyResponse(privateKey), getNonceCache())
+	require.NoError(t, err)
+}
+
 func TestParseLnurlpRequestUnsupportedVersion(t *testing.T) {
 	privateKey, err := secp256k1.GeneratePrivateKey()
 	require.NoError(t, err)

uma/uma.go

@@ -264,6 +264,18 @@ func IsUmaLnurlpQuery(url url.URL) bool {
 //
 //	url: the full URL of the uma request.
 func ParseLnurlpRequest(url url.URL) (*protocol.LnurlpRequest, error) {
+	return ParseLnurlpRequestWithReceiverDomain(url, url.Host)
+}
+
+// ParseLnurlpRequestWithReceiverDomain Parses the message into an LnurlpRequest object using an overridden receiver UMA domain.
+//
+// This is useful for cases where the receiver domain is not the same as the incoming request Host, for example when the
+// request is being proxied to another internal service.
+// Args:
+//
+//	url: the full URL of the uma request.
+//	receiverDomain: the domain of the receiver UMA of the payment. This is used to override the domain in the URL.
+func ParseLnurlpRequestWithReceiverDomain(url url.URL, receiverDomain string) (*protocol.LnurlpRequest, error) {
 	query := url.Query()
 	signature := query.Get("signature")
 	vaspDomain := query.Get("vaspDomain")
@@ -292,7 +304,7 @@ func ParseLnurlpRequest(url url.URL) (*protocol.LnurlpRequest, error) {
 	if len(pathParts) != 4 || pathParts[1] != ".well-known" || pathParts[2] != "lnurlp" {
 		return nil, errors.New("invalid uma request path")
 	}
-	receiverAddress := pathParts[3] + "@" + url.Host
+	receiverAddress := pathParts[3] + "@" + receiverDomain
 
 	nilIfEmpty := func(s string) *string {
 		if s == "" {