StoreGate: generalize parent container in DecorHandleKey

Frank Winklmeierrequested to merge
fwinkl/athena:decorhandle into master

Allow a VarHandleKey as parent container in decoration handles. This covers the use-case of decorating a container that is already defined as ReadHandleKey dependency.

Follow-up to !58618 (merged) and thanks @rbielski for the suggestion.

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