Add interactiveMode in proxy.

[xuezhaojun]

This allows users to interact with managed clusters without constructing a clusteradm command everytime.

➜  ~ clusteradm proxy kubectl --cluster=cluster1 --sa=test -i
Please enter the kubectl command and use "exit" to quit the interactive mode
kubectl> get nodes
NAME                     STATUS   ROLES                  AGE   VERSION
cluster1-control-plane   Ready    control-plane,master   51m   v1.21.1
kubectl> get pods
No resources found in default namespace.
kubectl> get serviceaccount
NAME      SECRETS   AGE
default   1         51m
kubectl> exit
Exit from interactive mode

[xuezhaojun]

/assign @qiujian16

[xuezhaojun]

The e2e-test is failing but should be solved in another PR.

[qiujian16]

this looks awesome!

[xuezhaojun]

@qiujian16 The e2e test is passed, PTAL. Thanks!

[qiujian16]

/approve
/lgtm

[openshift-ci]

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