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 -n as 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):

Vote on this issue!

This is an invitation to the Velero community to vote on issues, you can see the project's top voted issues listed here.
Use the "reaction smiley face" up to the right of this comment to vote.

• 👍 for "The project would be better with this feature added"
• 👎 for "This feature will not enhance the project in a meaningful way"

[kaovilai]

Thank you!

[kaovilai]

Related to #7566