New feature: support for no-reply methods

Some dbus methods do not expect a reply, and so zbus should support setting the NoReplyExpected bit in the message header for such calls so that the function returns without a dbus round-trip.

• This likely needs to be part of MessageBuilder
• The dbus_proxy macro should support it with an annotation, requiring a fdo::Result<()> return value.

Example org.freedesktop.DBus.Introspectable.Introspect snippet from com.canonical.dbusmenu:

    <method name="Event">
      <arg name="id" type="i" direction="in"/>
      <arg name="eventId" type="s" direction="in"/>
      <arg name="data" type="v" direction="in"/>
      <arg name="timestamp" type="u" direction="in"/>
      <annotation name="org.freedesktop.DBus.Method.NoReply" value="true"/>
    </method>