llama : add --completion-bash option (#11846)
This commit adds a new option `--completion-bash` to the llama.cpp which
outputs a source-able bash completion script.
The motivation for this change is to provide a more user-friendly
experience for users who use the command-line interface of llama.cpp.
This is currently only basic and all options are displayed for all llama
executables but this can be improved in the future if needed.
Example usage:
```console
$ build/bin/llama-cli --completion-bash > ~/.llama-completion.bash
$ source ~/.llama-completion.bash
$ ./build/bin/llama-server --m<TAB>
--main-gpu --mirostat --mirostat-lr --model --multiline-input
--min-p --mirostat-ent --mlock --model-url
```
