✨ Expose ManagedCluster annotations during join via clusteradm flag

[arturshadnik]

Summary

This PR adds a new optional flag --klusterlet-annotation to the clusteradm join command. With this flag, users can annotate ManagedClusters during the registration process.

Example usage:

clusteradm join --hub-token <token> \
--hub-apiserver <hub_apiserver_url> \
--cluster-name <cluster_name> \
--klusterlet-annotation key1=value1

Multiple annotations can be set by passing the flag multiple times:

clusteradm join --hub-token <token> \
--hub-apiserver <hub_apiserver_url> \
--cluster-name <cluster_name> \
--klusterlet-annotation key1=value1 \
--klusterlet-annotation key2=value2

Since Klusterlet requires all annotations to have a prefix of agent.open-cluster-management.io, the command is set up to automatically prefix any key which does not have it. This will simplify the command for end-users. Both of the following uses are valid and will result in the same outcome:

--klusterlet-annotation key1=value1
--klusterlet-annotation agent.open-cluster-management.io/key1=value1

kubectl get managedclusters cluster1 -o yaml --context kind-clusteradm-e2e-test-hub | grep annotations -A 2        
                 
    annotations:
      agent.open-cluster-management.io/key1: value1

This PR also adds an e2e test to ensure the annotations are correctly applied to the ManagedCluster.

Related issue(s)

Fixes #480

[qiujian16]

/approve
/lgtm

[openshift-ci]

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