Shania Twain sees her Glastonbury performance as a once-in-a-lifetime opportunity. The 58-year-old singer is set to play the Legend’s slot at the upcoming festival, and Shania is thrilled to be following in the footsteps of people like Dolly Parton, Diana Ross and Kenny Rogers. She told the BBC: “They are all people I consider to be music heroes and have been a big influence on my own career, so it’s surreal to have been invited.” Shania believes her Glastonbury performance will be a landmark moment in her career. She said: “It’s been explained to me that it is a real event, a once in a lifeti…