Add `manual_variables` field to `JobType`

What does this MR do and why?

This MR adds a manual_variables field to JobType, which returns the variables given to a manual job when it is triggered by a user. Builds that are not manual will always have an empty collection for this field. Bridges will have null.

How to set up and validate locally

1. Create a pipeline with a manual job

   manual_job:
     script:
       - echo "I am manual"
     when: manual

2. Run the pipeline and go to the job's page. Do not trigger the job from the pipeline graph

3. Add a variable using the form on the page, and then run the job

4. Go to /-/graphql-explorer and run this query:

   {
     project(fullPath: "PROJECT_PATH") {
       pipeline(id: "PIPELINE_ID") {
         jobs {
           nodes {
             manualVariables {
               key
               value
             }
           }
         }
       }
     }
   }

5. See that your variable is present in the return with the correct key and value

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #361236 (closed)