[CLI] Expose shell completions for `codeql` command

`codeql` doesn't support auto-completion which would be quite nice when working with it.

I would imagine a command like `codeql generate-completions` which can then be used as `source <(codeql generate-completions)`.

It shouldn't be too hard to implement this, because as far as I know, picocli is used for CLI parsing and it shouldn't take more than a few lines of code to implement this as the hard work has already been done by picocli:
https://picocli.info/autocomplete.html#_distribution
(All you have to do is add `@Command(name = "codeql", subcommands = GenerateCompletion.class)` it seems)