[IMP] whatsapp message id in conversation object

when user creates whatsapp conversation using client.conversation_start it doesn't returns
the message_id anywhere. conversation_start method actually creates and sends the whatsapp message,
there is no way to get the message id, this PR enables user to get the message id

Author: Sanjay Jamod

messagebird/conversation.py

@@ -79,6 +79,7 @@ def __str__(self):
             'id                   : %s' % self.id,
             'contact id           : %s' % self.contactId,
             'last used channel id : %s' % self.lastUsedChannelId,
+            'message id           : %s' % self.messages.lastMessageId,
             'message total count  : %s' % self.messages.totalCount,
             'status               : %s' % self.status,
             'created date time    : %s' % self.createdDatetime,

messagebird/conversation_message.py

@@ -57,6 +57,7 @@ class ConversationMessageReference(Base):
     def __init__(self):
         self.totalCount = None
         self.href = None
+        self.lastMessageId = None
 
 
 class ConversationMessageList(Base):