Description of Change(s)

• Introduces an _UnderlyingIterator type alias for Usd_PrimData* in Usd_PrimDataSiblingIterator and Usd_PrimDataSubtreeIterator
• Elides implementing operator-> for these internal types as it is currently not used. As the reference type is not a true reference, a spurious proxy class would be required to properly support pointer semantics.
• Explicitly implements the increment, dereference, equality and arrow operators for Usd_PrimDataSiblingIterator and Usd_PrimDataSubtreeIterator

Fixes Issue(s)

• Replace boost::iterator_adaptor usage with fully specified iterator classes #2228

• I have verified that all unit tests pass with the proposed changes

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