Support uninstalling velero using velero cli

[vadasambar]

Describe the problem/challenge you have
We have velero install ... to install velero in a cluster but we don't have a corresponding velero uninstall ... option.
To uninstall velero you have to manually delete the velero Deployment, namespace and CRDs (ref: #2567)

Describe the solution you'd like
I think it would be great if we could have velero uninstall ... option in the cli to automate the above.

Environment:

• Velero version (use velero version):

Client:
	Version: v1.5.3
	Git commit: 123109a3bcac11dbb6783d2758207bac0d0817cb
Server:
	Version: v1.5.3

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