[Feature] Fleet: Allow user to drain a cluster for all the resources placed by fleet

[ryanzhang-oss]

Is your feature request related to a problem? Please describe.
User wants to drain all the workloads from a cluster before upgrading it.

Optional: Set the appropriate GitHub label(s) to indicate the AKS feature or area your feature request relates to.

Describe the solution you'd like
A simple command similar to kubectl drain node to drain a cluster

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[microsoft-github-policy-service]

@sjwaight would you be able to assist?