Replace boost::iterator_facade with explicit implementation for SdfMapProxy

[nvmkuruc]

Description of Change(s)

• Replace iterator_facade with explicit implementation for SdfMapProxy
• Uses type traits / SFINAE to provide two implementations of operator-> depending on whether the reference type is a true reference and can be safely converted to a pointer or if it's a proxy type and needs a pointer proxy to manage lifetime. Added a comment to recommend deprecating usage of operator-> for C++20 to avoid this complexity.

Fixes Issue(s)

• Replace boost::iterator_facade usage with fully specified iterator classes #2305

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

• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-8522