Create RunRequest payload from CI job

Problem

A CI/CD job request needs to be translated to a Steps job request. Solving this technical issue is required to enable features such as Execute CI job via Step Runner service - Kubern... (#37450 - closed) and Execute CI job via Step Runner service - Docker... (#37414 - closed).

Proposal

Given a (legacy) CI job, create a RunRequest payload according to https://docs.gitlab.com/ee/architecture/blueprints/gitlab_steps/runner-integration.html#runrequest-parameters. RunRequest.Steps should be a JSON-serialized version of https://gitlab.com/gitlab-org/step-runner/-/tree/main/schema/v1. This includes populating RunRequest.Job.