Support variables in rules: exists

What does this MR do and why?

Support variable expansion in rules: exists. This change is under the feature flag ci_variable_expansion_in_rules_exists.

Screenshots or screen recordings

When the rules match:

variables:
  PATTERN: '*.yml'
  WRONG_PATH: 'wrong.rb'
rule_exist_job:
 script: 
    - echo $PATTERN
    - echo $WRONG_PATH
    - echo $EXIST_PATH
 rules:
    - exists:
      - $PATTERN

In the example above, we have an existing deploy.yml file, so the rule_exist_job runs:

When the rules do not match:

variables:
  PATTERN: '*.yml'
  EXIST_PATH: 'upload.yml'
  WRONG_PATH: 'wrong.rb'
rule_exist_job:
 script: 
    - echo $PATTERN
    - echo $WRONG_PATH
    - echo $EXIST_PATH
 rules:
    - exists:
      - $PATTERN

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MR acceptance checklist

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

Related to: #283881 (closed)
Rollout issue: #381046 (closed)