Support all Allocate flags in UnknownArrayHandle

`UnknownArrayHandle` supported an `Allocate` method to change
the size of the underlying array without knowing its type.
However, it did not give all the features of `ArrayHandle`'s
allocate. Namely, you could not specify that the data should
be preserved and you could not provide a `Token` object. This
change adds these (optional) parameters.
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