Replace boost::iterator_adaptor with explicitly specified iterator definition for UsdPrim{Sibling,Subtree}Iterator

[nvmkuruc]

Description of Change(s)

• Introduces an _UnderlyingIterator type alias for const Usd_PrimData* in UsdPrimSiblingIterator
• Introduces an _UnderlyingIterator type alias for const UsdPrimDataConstPtr in UsdPrimSubtreeIterator
• Introduces _PtrProxy private classes to be used as the pointer type for UsdPrimSiblingIterator and UsdPrimSubtreeIterator. This special proxy is required because their reference type aliases are not true reference types and it's not safe to take the address of a temporary.
• Explicitly implements the increment, dereference, equality and arrow operators for UsdPrimSiblingIterator and UsdPrimSubtreeIterator
• Fix doxygen string for pre/post increment operators

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

• I have submitted a signed Contributor License Agreement

[tallytalwar]

Filed as internal issue #USD-7962