Add support to user defined variable for manual_confirmation

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What does this MR do and why?

Add support to user defined variable for manual_confirmation MR acceptance checklist

Connected to #468971

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Add .gitlab-ci.yml with this context

   variables:
     GLOBAL_VAR: "A global variable"
   
   stages:
     - build
     - test
   
   build-job:
     stage: build
     variables:
       JOB_BUILD_VAR: "A job variable"
     script:
       - echo "Variables are '$GLOBAL_VAR' and '$JOB_BUILD_VAR'"
     when: manual
     manual_confirmation: "Variables are '$GLOBAL_VAR' and '$JOB_BUILD_VAR'"
   
   test-job:   
     variables:
       JOB_TEST_VAR: "A job variable"
     script:
       - echo "Variables are '$GLOBAL_VAR' and '$JOB_TEST_VAR'"
     when: manual
     manual_confirmation: "Variables are '$GLOBAL_VAR' and '$JOB_TEST_VAR'"

2. Manual confirmation modal should be replaced by variables.