Add performance profiling suite

[corybarr]

The PR adds a new "Performance Tracing Suite" app to the extension. When run, it will run performance tracing on every USDC file in the tests/testAssets/usd/flattened directory. One JSON trace analysis file is output for every USDC file. They are intended to be used in CI to query summary statistics.

This app extends the Performance Tracing app, which starts at launch and concludes when the app is closed. This new app will stop the at-launch profiling and write it to disk before profiling each USDC file. There will be one more profiling file than the number of USDC files.

Further work needs to be done to make better use of the trace analyses:

• Integrate with CI
• Generate reports or otherwise make available for querying of summary statistics
• Improved USDC files

Optional uses include the following

• Gain profiling reproducibility by adding UI to select a test file for playback when doing live profiling with Nsight, Superluminal, etc.