feat: Add support for message pacts (fixes #7), add example

example/lib/main.dart

@@ -0,0 +1,80 @@
+import 'package:http/http.dart' as http;
+import 'package:pact_dart/pact_dart.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('HTTP consumer example', () {
+    late PactMockService pact;
+
+    setUp(() {
+      pact = PactMockService('HTTP consumer', 'Some API');
+    });
+
+    tearDown(() {
+      return pact.reset();
+    });
+
+    test('get users', () async {
+      pact
+          .newInteraction()
+          .given("a user exists",
+              params: {'first_name': 'Betsy', 'last_name': 'Tester'})
+          .uponReceiving('a request for all users')
+          .withRequest('GET', '/users')
+          .willRespondWith(200, body: {
+            // Matchers are used here as we care about the types and structure of the response and not the exact values.
+            'page': PactMatchers.SomethingLike(1),
+            'per_page': PactMatchers.SomethingLike(20),
+            'total': PactMatchers.IntegerLike(20),
+            'total_pages': PactMatchers.SomethingLike(3),
+            'data': PactMatchers.EachLike([
+              {
+                'id': PactMatchers.uuid('f3a9cf4a-92d7-4aae-a945-63a6440b528b'),
+                'first_name': PactMatchers.SomethingLike('Betsy'),
+                'last_name': PactMatchers.SomethingLike('Tester'),
+                'salary': PactMatchers.DecimalLike(125000.00)
+              }
+            ])
+          });
+
+      pact.run(secure: false);
+
+      await http.get(Uri.parse('http://${pact.host}:${pact.port}/users'));
+
+      pact.writePactFile();
+    });
+  });
+
+  group('message consumer example', () {
+    test('payment rejected', () async {
+      final pact = MessagesPact('message consumer', 'message provider');
+      await pact
+          .newMessage()
+          .given("a user exists", params: {'id': 'user_id'})
+          .andGiven('payment is rejected')
+          .expectsToReceive('payment rejected message')
+          .withContent({
+            'type': 'payment rejected',
+            'payment_id':
+                PactMatchers.uuid('f3a9cf4a-92d7-4aae-a945-63a6440b528b'),
+          })
+          .withMetadata({'foo': 'bar'})
+          .verify((message, metadata) {
+            expect(
+                message,
+                equals({
+                  'type': 'payment rejected',
+                  'payment_id': 'f3a9cf4a-92d7-4aae-a945-63a6440b528b'
+                }));
+            expect(
+                metadata,
+                equals({
+                  'contentType': 'application/json',
+                  'foo': 'bar',
+                }));
+          });
+
+      pact.writePactFile();
+    });
+  });
+}