[CLI] Expose shell completions for `codeql` command

[intrigus-lgtm]

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)

[smowton]

I'll pass the feature request along!

[smowton]

I had a quick tinker with this and found that the simple approach listed above doesn't work for us -- (a) it generates a script incompatible with macOS bash, and (b) it generates a script that seems unaware of any completions; I guess the way we specify subcommands etc isn't sufficiently obvious.