A bipartisan group of 50 Beacon Hill lawmakers has signed a letter demanding the state Department of Correction appeal a federal judge’s ruling that would give a man convicted of killing his wife a sex-change operation at taxpayer expense. “Like many others, I was shocked at the extreme interpretation of the Eighth Amendment,” said Senate Minority Leader Bruce Tarr, who wrote the letter co-signed by 49 other state senators and representatives. “We can’t subject our prisoners to cruel and unusual punishment. We have to provide them with medically necessary treatment. But that’s a completely radical treatment and one that many people outside of prison cannot afford or obtain.”