By David Ljunggren OTTAWA (Reuters) - Canada's main opposition parties signaled on Thursday that they might oppose government plans to send fighter jets to the Middle East, putting more pressure on Prime Minister Stephen Harper as he struggles in the polls ahead of an election next year. Harper is expected to an**unce on Friday that Ottawa wants to send at least six CF-18 fighter jets to participate in a U.S.-led campaign against Islamic State militants who have taken over large parts of Iraq and Syria. ...