Add variables argument to `JobPlay`mutation

What does this MR do and why?

We are transitioning the manual job page to GraphQL, so we now need to make it possible to play a job with a given set of variables via the JobPlay mutation.

Screenshots or screen recordings

With the following manual job:

manual_job:
    script:
        - echo $MANUAL_VAR_1
        - echo $MANUAL_VAR_2
    when: manual

Running the JobPlay mutation with variables:

Plays the job, which outputs the variable values:

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #386272 (closed)