GraphQL API: Add `projects` field to CiRunner
Related to #347856 (closed).
Background
In order to implement #347856[View_specific_runner.png], we should provide the projects of a given runner in our GraphQL API.
![#347856[View_specific_runner.png]](https://gitlab.com/gitlab-org/gitlab/-/issues/347856/designs/View_specific_runner.png){kind=link}
Implementation
In #345348 (comment 731608888) we described a way to define a "main" project that owns the runner, solving ownership issues related to the project.
We could consider addressing setting an "owner" or "main" project for specific runners at the database level before starting work on the API.
Description
Add a 2 nullable "project" / "associatedProjects" to the runner.
Edited by Pedro Pombeiro