Add job_variables_attributes to play build API

What does this MR do and why?

This MR adds an optional job_variables_attributes param to the REST API endpoint for playing manual jobs. Users can pass custom variables to the param in order to have those variables set on the job.

Implements: #37267 (closed)

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Set up a pipeline with a manual job that includes script: echo "$TEST_VAR"
2. Run a pipeline
3. Navigate to the manual job's show page and get the job ID
4. Run curl --request POST --header "PRIVATE-TOKEN: <private_access_token>" "<gdk.test>/api/v4/projects/<project_id>/jobs/<job_id>/play" -d "job_variables_attributes%5B%5D%5Bkey%5D=TEST_VAR&job_variables_ attributes%5B%5D%5Bvalue%5D=it+works%21"
5. See that your manual job has run and printed it works!

MR acceptance checklist

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