Replace boost::totally_ordered with explicit operator overloads for SdfAssetPath

[nvmkuruc]

Description of Change(s)

• Add explicit implementations of operator{<=,>,>=,!=} to SdfAssetPath
• Expose copy constructor to python
• Expose comparison operators to python
• Add unit test coverage for Sdf.AssetPath operators

Fixes Issue(s)

• boost::totally_ordered can be replaced with explicit operator overloads #2250

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

• I have submitted a signed Contributor License Agreement

[sunyab]

Filed as internal issue #USD-8030

[crevilla]

I'm glad to see test cases added for SdfAssetPath, but I don't feel they belong in testSdfReference (it's not obvious to look for them here.) Can we instead add a new testSdfAssetPath to hold these tests? I'm a little surprised we don't have a testSdfAssetPath already.
Additionally, we typically make one unittest.TestCase class and have multiple test case methods for each case. That would make more sense here too for the trio of SdfAssetPath tests.