Need a confirm flag for velero plugin add CLI

[danfengliu]

Describe the problem/challenge you have In E2E test, velero plugin add CLI is involved in additional BSL test, so we need a-y/-n flag and -nas the default input for this CLI so that no Human-Computer Interaction is needed for automatic tests.

installPluginCmd cmd =/velero/workspace/velero-e2e-test/e2e/velero/test/e2e/../../_output/bin/linux/amd64/velero --namespace velero plugin add velero/velero-plugin-for-aws:main
velero plugin add may cause the Velero server pod restart, so it is a dangerous operation
once Velero server restarts, all the ongoing jobs will fail.
Are you sure you want to continue (Y/N)? error reading user input: EOF

Describe the solution you'd like

Anything else you would like to add:

Environment:

• Velero version (use velero version):
• Kubernetes version (use kubectl version):
• Kubernetes installer & version:
• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release):

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[kaovilai]

Thank you!

[kaovilai]

Related to #7566