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

[nvmkuruc]

Description of Change(s)

• Explicitly implement the increment, dereference, equality, and arrow operators for VtDictionary::iterator
• Uses nullptr instead of 0 as the value to represent a null map per the C++ Core Guidelines (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-nullptr)

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-7961