mediawiki/api/request: Add context.ab_test field
Why?
Test Kitchen adds experiment assignments - a map of experiment name to treatment group name - to the global logging context. This context is included in the events that are produced to the mediawiki.api_request stream. At the time of writing, this is resulting in 600+ validation errors/min on that stream*
Bug: T411356
* I underestimated the validation error rate in the original commit message