Added WebpushNotification.Builder API with additional Webpush parameters (#193)

* Added webpush notification fields

* Added documentation

* Support for adding arbitrary key-value pairs to WebpushNotification

* Updated changelog

* Added a couple of helper methods for clarity

* Updated comment

* Renamed setTimestamp to setTimestampMillis as per API review feedback

Author: hiranya911

CHANGELOG.md

@@ -1,6 +1,7 @@
 # Unreleased
 
--
+- [added] `WebpushNotification` type now supports arbitrary key-value
+  pairs in its payload.
 
 # v6.3.0
 

src/main/java/com/google/firebase/messaging/WebpushConfig.java

@@ -35,12 +35,12 @@ public class WebpushConfig {
   private final Map<String, String> data;
 
   @Key("notification")
-  private final WebpushNotification notification;
+  private final Map<String, Object> notification;
 
   private WebpushConfig(Builder builder) {
     this.headers = builder.headers.isEmpty() ? null : ImmutableMap.copyOf(builder.headers);
     this.data = builder.data.isEmpty() ? null : ImmutableMap.copyOf(builder.data);
-    this.notification = builder.notification;
+    this.notification = builder.notification != null ? builder.notification.getFields() : null;
   }
 
   /**