istioctl allow bug-report to specify output dir/file

[steeling]

(This is used to request new product features, please visit https://discuss.istio.io for questions on using Istio)

Describe the feature request

What: Add a flag to istioctl bug-report to specify output location for the generated archive.

Background: Currently with istioctl bug-report you can specify the temp dir to download the artifacts before creating the archive, which will always be created in the local dir with the name bug-report.tar.gz. There is no way to override this.

Why? We require a method to run istioctl from within a pod itself and may invoke multiple runs simultaneously against multiple meshes.

Describe alternatives you've considered

Wrapping istioctl in a shell script.