Some artists might consider a musical theater role portraying a legend like Frank Sinatra to be A potential career killer because of the comparisons to Ol’ Blue Eyes, and then the prospect of not being able to get out from under that shadow and assert your own individuality as a singer. That, however, is not the situation with Jonathan Poretz, a jazz vocalist hailing from the San Francisco area who has spent almost two years depicting Sinatra in the production of Dick Feeney and Sandy Hackett …