Replace boost::variant with std::variant in sdf

[nvmkuruc]

Description of Change(s)

This replaces boost::variant with std::variant in sdf. The API uses boost::bad_get to communicate a failure to parse due to a bad variant access or an out of range error. This change remaps the STL std::bad_variant_access to a boost::bad_get to preserve this behavior. As visitors used via std::visit do not specify result_type, the return type for several helper methods has been converted to auto.

One or more follow up changes will be needed to remove a spurious include of boost/variant.hpp from the parser logic and replace boost::bad_get with a new proposed Sdf_BadGet exception.

Fixes Issue(s)

• 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-8739