Allow adding labels to ManagedCluster during registration

[arturshadnik]

Describe the enhancement
Similar to the ability to add annotations to a ManagedCluster by setting klusterletChartConfig.Klusterlet.RegistrationConfiguration.ClusterAnnotations when creating a Klusterlet, it would be very useful to be able to add labels in the same way.

One usecase for this is the ability to automatically assign/exclude new ManagedClusters to a ManagedClusterSet by using label selectors. Right now, a manual step is required to add the desired label after the ManagedCluster is created.

[qiujian16]

cc @zhiweiyin318

[arturshadnik]

Please also consider propagating new labels in Klusterlet spec after registration, same as what is mentioned in #1210

[qiujian16]

Agent does not have the permission to update metadata and spec of the managedcluster, it can only update status. What is the use case you want to achieve by letting agent update labels of the managedcluster?