Add a command argument for choosing shell

[twobiers]

This PR adds a positional argument to the InstallCompletions command for choosing the shell.
This allows e.g. to put source <$(./cli completions bash) in ~/.bashrc and source <$(./cli completions zsh) into ~/.zshrc

Tested locally with the Crossplane CLI:

# zsh is my login shell
$ crossplane completions
autoload -U +X bashcompinit && bashcompinit
complete -C /Users/thund/Repos/privat/crossplane/crossplane crossplane

$ crossplane bash
complete -C /Users/thund/Repos/privat/crossplane/crossplane crossplane

$ crossplane zsh
autoload -U +X bashcompinit && bashcompinit
complete -C /Users/thund/Repos/privat/crossplane/crossplane crossplane

$ crossplane fish
function __complete_crossplane
    set -lx COMP_LINE (commandline -cp)
    test -z (commandline -ct)
    and set COMP_LINE "$COMP_LINE "
    /Users/thund/Repos/privat/crossplane/crossplane
end
complete -f -c crossplane -a "(__complete_crossplane)"

$ crossplane pwsh
Usage: crossplane completions [<shell>] [flags]

Get shell (bash/zsh/fish) completions. You can source this command to get
completions for the login shell. Example: 'source <(crossplane completions)'

Arguments:
  [<shell>]    The shell to install completion for. If not provided, the user's
               login shell will be used.

Flags:
  -h, --help         Show context-sensitive help.
      --verbose      Print verbose logging statements.

      --uninstall

crossplane: error: unsupported shell pwsh