By Andrew MacAskill, Kate Holton and Alistair Smout LONDON (Reuters) -British Prime Minister Rishi Sunak appealed to his Conservative lawmakers on Thursday to unite behind his plan to send asylum seekers to Rwanda after a revolt that once again exposed deep divisions in his party. Sunak is facing the biggest challenge to his year-long tenure as he tries to stop lawmakers on the party’s right wing from rebelling over their demand that Britain should quit international treaties to set its own migration policy. His immigration minister quit on Wednesday and he is facing questions as to whether he…