Add USER-AGENT header in Github tarball retrieving

[Jeffwan]

Resolve this. https://github.com/kubeflow/kubeflow/issues/5245

Seems some corp network has restrictions on the requests. Setting dedicate user-agent solve the problem. It would be better to include kfctl version here. However, version is in cmd package and if we import it, we will see error like beflow. We can leave it to kfctl at this moment since no server side application need that information

../../pkg/kfconfig/types.go:10:2: import "github.com/kubeflow/kfctl/v3/cmd/kfctl" is a program, not an importable package
import cycle not allowed
package main
    imports github.com/kubeflow/kfctl/v3/cmd/kfctl/cmd
    imports github.com/kubeflow/kfctl/v3/pkg/kfapp/coordinator
    imports github.com/kubeflow/kfctl/v3/pkg/kfapp/aws
    imports github.com/kubeflow/kfctl/v3/pkg/kfconfig
    imports github.com/kubeflow/kfctl/v3/cmd/kfctl
    imports github.com/kubeflow/kfctl/v3/cmd/kfctl/cmd

[kubeflow-bot]

This change is 

[PatrickXYS]

/lgtm

[crobby]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: crobby

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubeflow/kfctl/blob/master/OWNERS)~~ [crobby] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[PatrickXYS]

http://testing-argo.kubeflow.org/workflows/kubeflow-test-infra/kubeflow-kfctl-presubmit-unittests-401-abc9dfb-7600-9acb?tab=workflow&nodeId=kubeflow-kfctl-presubmit-unittests-401-abc9dfb-7600-9acb-851443497

Looks like test is still ongoing, guess it's temp error.

/test kubeflow-kfctl-presubmit

[Jeffwan]

/test kubeflow-kfctl-presubmit