Support for a --outfile flag for kubelogin convert-kubeconfig

[zawachte]

As a user, I would like to use kubelogin convert-kubeconfig to generate and edit multiple kubeconfigs for members of my team, pipelines and other services that may need a kubeconfig.

It would be very helpful if we can add a --outfile to convert-kubeconfig to have a way to easily generate new kubeconfigs.

As a workaround I can use:

cp kubeconfig kubeconfig-new
kubelogin convert-kubeconfig --kubeconfig kubeconfig-new

but something native would be nice.