Reintroduce shell completion command

[vigsterkr]

#2512 removed click_completion i.e renku --install-completion command due to the fact that click_completion is not working with click 8.x anymore.

It would be still nice to have a command that basically provides the user the completion command for $SHELL, that even could be interpreted, so that way adding it to the shell config file something like renku --shell-integration would activate the shell completion functionality