Implement a ReAct prompt on AI Gateway

Problem

As a part of https://gitlab.com/gitlab-org/gitlab/-/issues/446206+ we want to have better vision how the whole system will work together. We need a proof of concept of changes required on the gateway, then we can better understand how to implement other modules.

Proposal

1. Implement a new endpoint on AI Gateway that will implement the ReAct prompt assembling. It should be similar to an inner loop of ZeroShot::Executor in monolith.
2. New endpoint should accept version of GitLab to be able to filter tools accessible in this version.
3. We need to have some mechanism for versioning tools descriptions, but it's outside of the scope of this issue. For the sake of this PoC we can skip it and pretend that the is only one version.