Support DRMAA API for external CI scheduler

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In some cases, a company may like to implement a more advanced scheduler/prioritization system for handling a shared runner queue. There is an existing standard API for this, DRMAA: https://en.wikipedia.org/wiki/DRMAA

We should consider supporting this as a method of advanced scheduling for customers who need it, without having to build all of the complexity desired within GitLab ourselves. (Which may or may not ultimately end up meeting our users needs)