Add saved payout destination UX and service contracts (#54)

frontend/packages/sxchnge_wallet/test/two_factor_test.dart

@@ -27,6 +27,15 @@ void main() {
             amount: 1000,
             destination: kind == TransferKind.exchange ? 'usd' : null,
             recipient: 'Sample recipient',
+            payoutDestination: OneTimeDestination(
+              CryptoDestination(
+                assetId: 'demo:ethereum:native',
+                networkId: 'eip155:1',
+                address: '0x1234567890123456789012345678901234567890',
+                assetLabel: 'ETH',
+                networkLabel: 'Ethereum',
+              ),
+            ),
           ),
         );
         await expectLater(
@@ -59,6 +68,15 @@ void main() {
         source: 'eur',
         amount: 1000,
         recipient: 'Sample recipient',
+        payoutDestination: OneTimeDestination(
+          CryptoDestination(
+            assetId: 'demo:ethereum:native',
+            networkId: 'eip155:1',
+            address: '0x1234567890123456789012345678901234567890',
+            assetLabel: 'ETH',
+            networkLabel: 'Ethereum',
+          ),
+        ),
       ),
     );
     final c = (await factor.begin(FactorPurpose.payout, q.id))!;
@@ -85,6 +103,15 @@ void main() {
         source: 'eur',
         amount: 1000,
         recipient: 'Sample recipient',
+        payoutDestination: OneTimeDestination(
+          CryptoDestination(
+            assetId: 'demo:ethereum:native',
+            networkId: 'eip155:1',
+            address: '0x1234567890123456789012345678901234567890',
+            assetLabel: 'ETH',
+            networkLabel: 'Ethereum',
+          ),
+        ),
       ),
     );
     final c = (await factor.begin(FactorPurpose.payout, 'different-quote'))!;