Remove boost::recursive_wrapper usage from JsValue

[nvmkuruc]

Description of Change(s)

It's expected that upon adoption of C++17, USD will replace boost::variant with std::variant. However, std::variant does not support boost::recursive_wrapper and does not have an equivalent concept.

This PR introduces Js_Wrapper, internally leveraging std::unique_ptr, but adds the invariant that it is always dereferenceable (ie. always non-NULL) and implements equality operators via the dereferenced value not the pointer value.

Js_Wrapper can then be used to add a level of indirection to JsValue::_Holder for types (JsObject and JsArray) that may contain JsValues.

This PR adds test coverage for an array of objects nested in an object to validate both conversion and equality testing for more complicated structures.

Fixes Issue(s)

N/A

• 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-8132