With the Knicks set to tip off the Eastern Conference finals on Tuesday , legendary center Patrick Ewing takes a shot at some playoff Q&A with Post columnist Steve Serby. Q: What would you tell these Knicks if you were standing in the locker room about the opportunity that is in front of them right now? A: What I would tell them is they have a once-in-a-lifetime opportunity. No one knows if it’s going to come back around. When we got there in ’94, I definitely thought that we were going to have