adding auth token in header for listbalances client method. fixed tests

lib/src/http_client/tbdex_http_client.dart

@@ -153,14 +153,17 @@ class TbdexHttpClient {
   }
 
   static Future<List<Balance>> listBalances(
+    BearerDid did,
     String pfiDid,
   ) async {
+    final requestToken = await _generateRequestToken(did, pfiDid);
+    final headers = {'Authorization': 'Bearer $requestToken'};
     final pfiServiceEndpoint = await _getPfiServiceEndpoint(pfiDid);
     final url = Uri.parse('$pfiServiceEndpoint/balances/');
 
     http.Response response;
     try {
-      response = await _client.get(url);
+      response = await _client.get(url, headers: headers);
 
       if (response.statusCode != 200) {
         throw ResponseError(

test/http_client/tbdex_http_client_test.dart

@@ -139,28 +139,37 @@ void main() async {
 
     test('can list balances', () async {
       when(
-        () => mockHttpClient.get(Uri.parse('$pfiServiceEndpoint/balances/')),
+        () => mockHttpClient.get(
+          Uri.parse('$pfiServiceEndpoint/balances/'),
+          headers: any(named: 'headers'),
+        ),
       ).thenAnswer(
         (_) async => http.Response(TestData.listBalancesResponse(), 200),
       );
 
-      final response = await TbdexHttpClient.listBalances(pfiDid);
+      final response = await TbdexHttpClient.listBalances(TestData.aliceDid, pfiDid);
       expect(response.length, 1);
 
       verify(
-        () => mockHttpClient.get(Uri.parse('$pfiServiceEndpoint/balances/')),
+        () => mockHttpClient.get(
+          Uri.parse('$pfiServiceEndpoint/balances/'),
+          headers: any(named: 'headers'),
+        ),
       ).called(1);
     });
 
     test('list balances throws ResponseError', () async {
       when(
-        () => mockHttpClient.get(Uri.parse('$pfiServiceEndpoint/balances/')),
+        () => mockHttpClient.get(
+          Uri.parse('$pfiServiceEndpoint/balances/'),
+          headers: any(named: 'headers'),
+        ),
       ).thenAnswer(
         (_) async => http.Response('Error', 400),
       );
 
       expect(
-        () async => TbdexHttpClient.listBalances(pfiDid),
+        () async => TbdexHttpClient.listBalances(TestData.aliceDid, pfiDid),
         throwsA(isA<ResponseError>()),
       );
     });