* Julian Assange open to political action as Cannes hosts documentary WikiLeaks founder makes surprise showing at festival * ‘The Six Billion Dollar Man’ premieres at Cannes * Documentary highlights Assange’s extradition fight By Hanna Rantala and Miranda Murray CANNES, France, – WikiLeaks founder Julian Assange, who is at the Cannes Film Festival this week for the documentary “The Six Billion Dollar Man,” is thinking about how to become politically active again once he has fully recovered from prison, said his wife, Stella. Assange, 53, returned to his native Australia after pleading guilty last June under an agreement with U.S. officials to one count of illegally obtaining and disclosing national security materials. The plea ended Assange’s five-year stay in a British prison, which followed seven years at the Ecuador embassy as he sought to avoid extradition to Sweden on sexual assault allegations. Assange denied those allegations and called them a pretext to extradite him to the United States over WikiLeaks. WikiLeaks in 2010 released hundreds of thousands of classified U.S. military documents on Washington’s wars …