Updates Config::Entry::Variable value to support array

What does this MR do and why?

• Make Config::Entry::Variable value support an array of values for a variable
• Add allow_array metadata to the variables keyword in Entry::Root to only allow top-level variables to have array values.
• Update the value field for ConfigVariableType to support multiple values. Since GraphQL does not support scalar unions, we are going to support either an array of stings or a string wrapped in an array.

Screenshots or screen recordings

GraphQL:

query {
  project(fullPath: "root/variables-test"){
    ciConfigVariables(sha:"0dbda96f0c5a5979c80dd56325f2a85c4e92f349"){
    	description
      key
      value
    }
  }
}

{
  "data": {
    "project": {
      "ciConfigVariables": [
        {
          "description": "The deployment target. Change this variable to 'canary' or 'production' if needed.",
          "key": "DEPLOY_ENVIRONMENT",
          "value": ["staging", "Production"]
        }
      ]
    }
  }
}

NOTE: This work is backend support for frontend: !96542 (merged), which is under the run_pipeline_graphql feature flag.

MR acceptance checklist

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