This task covers:
* Implementing Recommendation #4: Expose Experiment Enrollment Sampling from {T408133}
=== AC
[] Developers can query the experiment enrollment status for a given experiment and identfier
[] Developers can make those queries regardless of whether the `MetricsPlatform.XLab.ExperimentManager` service has been initialized, e.g.
** Inside a maintenance script
** Inside a MediaWiki REST API handler
=== Notes
1. Psuedocode from T408133:
```lang=php
namespace MediaWiki\Extension\MetricsPlatform\TestKitchen;
interface Coordinator {
/**
* Gets the experimental group assignment for the identifier.
*
* This method is pure – it has no side effects.
*
* @return {string|null} If the user is enrolled in the experiment, then
* then the name of the experimental group; otherwise, `null`
*/
public function getEnrollmentForIdentifier( string $experimentName, string $identifier ): ?string;
}
```