Circular <scp>RNA</scp> nuclear receptor interacting protein 1 promoted biliary tract cancer <scp>epithelial‐mesenchymal transition</scp> and stemness by regulating the <scp>miR</scp> ‐515‐5p/ <scp>AKT2</scp> axis and <scp>PI3K</scp> / <scp>AKT</scp> / <scp>mTOR</scp> signaling pathway