Change UnboundedSourceAsSdfWrapperFn to share the cache across instances.

[scwhittle]

Add a utility class to enable sharing across all deserialized instances of a DoFn and use it in UnboundedSourceAsSdfWrapperFn to cache Readers across dofn instances.

fixes #32968

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[scwhittle]

R: @kennknowles

I don't really like the utility class name, let me know if you have some better idea. It seems like this could be a common use within DoFns. For example, @sjvanrossum made me think of this concern in his work on Kafka source in #32986

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