Expose experiment context_keys in the experiments API

What does this MR do and why?

Exposes each experiment's declared context_keys on the GET /experiments list API, via a new top-level context array on the Experiment entity.

Previously the API serialized the Feature::Definition but not the experiment class's context_keys, so consumers had no way to know which context an experiment expects (user, namespace, project, actor). As a result the Experiment Dashboard could only force/read/clear assignments for the current user. This unblocks it to target arbitrary users/namespaces/projects.

How to set up and validate locally

1. As a GitLab team member (SaaS mode), call:

   curl --header "PRIVATE-TOKEN: <token>" \
     "http://gdk.test:3000/api/v4/experiments"

2. Each item includes a context array — for example "context": ["user"] for null_hypothesis, and "context": [] for experiment flags without a registered class.

References

Closes #607813 (closed)

Related to #591174 (closed)

Related to https://gitlab.com/gitlab-org/growth/experiment-dashboard/-/work_items/5

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