Replace boost::iterator_adaptor with explicitly specified iterator definition for UsdPrimRange

[nvmkuruc]

Description of Change(s)

• Introduces an _UnderlyingIterator type alias for Usd_PrimDataConstPtr in UsdPrimRange::iterator
• Introduces a _PtrProxy private class to be used as the pointer type for UsdPrimRange::iterator. This special proxy is required because UsdPrimRange::reference is not a true reference type and it's not safe to take the address of a temporary.
• Explicitly implement the increment, dereference, and arrow operators for UsdPrimRange::iterator

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-7960