NestedTreeControl getChildren should accept a readonly return type

[jinder]

The getChildren parameter in the NestedTreeControl has the following signature:

 (dataNode: T) => (Observable<T[]> | T[] | undefined | null)

As the control does not manipulate the child collection, this should be changed to readonly so that it can be used with readonly arrays:

(dataNode: T) => (Observable<readonly T[]> | readonly T[] | undefined | null)

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