Add GraphQL query for inherited CI variables

What does this MR do and why?

Main issue: #222367 (closed)

This returns the CI variables a project inherits from its parent and ancestor groups. The query will be used for showing the inherited CI variables in the project's CI/CD settings (which is an existing table that will be migrated to vue in !119026 (merged)).

Screenshots or screen recordings

How to set up and validate locally

1. Tests should pass.
2. You can use the following query to test in graphiql.

Sample query

query getInheritedCiVariables {
  project(fullPath: "group/project") {
    id
    inheritedCiVariables {
      __typename
      nodes {
        id
        variableType
        environmentScope
        masked
        protected
        raw
        groupName
        groupCiCdSettingsPath
        __typename
      }
    }
  }
}

MR acceptance checklist

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