Adds CiConfigVariableType

What does this MR do and why?

• Adds the ciConfigVariable Type to GraphQL. This is the query, although it won't show up in GraphiQL because the service uses ReactiveCache.
• This also moves the id field out of VariableInterface and into specific variable types, since ciConfigVariables don't have id.

query {
  project(fullPath: "root/variables"){
    ciConfigVariables(sha:"efc2bd3cc3eaed2431de2761c5c326c25bd03368"){
    	value
    }
  }
}

This is MR 1/4 for: #362934 (closed)

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