[Feature] Add support for Test Optimization (preferably with a PHPUnit integration)

[mollierobbert]

Currently PHP does not seem supported for the Test Optimization feature. Meaning for PHP there does not seem to be a way to trace a PHPUnit test run.

I would love to see support for it.

At minimum the dd-trace-php lib could offer access to the raw Test Optimization API, so the community could build early versions of a PHPUnit integration.