app: add llama update self updater (#23865)
* wip: llama update POC
* cleaning: llama update
* llama-gen-docs
* app: delegate llama update to the install script
* app: spawn the installer detached so llama update can replace a running binary
* cleaning: inline llama update into llama.cpp, drop app-update.{cpp,h}
* app: make llama_update static
Address review from @angt
