examples : add passkey test (#3856)
* examples : add passkey test
* passkey : better prints
* passkey : select pass key pos from CLI
* passkey : simplify n_past logic
* make : add passkey target
* passkey : add "self-extend"-like context extension (#4810)
* llama : "self-extend"-like context extension
* passkey : add comment
* passkey : add readme
