implement Dbus sync methods (#737)

implement two Dbus methods:
- sendContacts
- sendSyncRequest

update documentation

src/main/java/org/asamk/signal/dbus/DbusSignalImpl.java

@@ -202,6 +202,24 @@ public class DbusSignalImpl implements Signal {
         }
     }
 
+    @Override
+    public void sendContacts() {
+        try {
+            m.sendContacts();
+        } catch (IOException e) {
+            throw new Error.Failure("SendContacts error: " + e.getMessage());
+        }
+    }
+
+    @Override
+    public void sendSyncRequest() {
+        try {
+            m.requestAllSyncData();
+        } catch (IOException e) {
+            throw new Error.Failure("Request sync data error: " + e.getMessage());
+        }
+    }
+
     @Override
     public long sendNoteToSelfMessage(
             final String message, final List<String> attachments