Most tennis fans can only dream about receiving the kind of thick, cream-colored invitation that State Senator Tony Avella of Queens got recently in the mail. The United States Tennis Association was pleased to invite him and a guest to view this year’s tennis championships, which begin Aug. 27. They could sit in the President’s Suite. They could pick “one session of your choice” during the first week. This is tennis at its best, viewed from the best seats in Arthur Ashe Stadium.