Add VariableType and variables fields

What does this MR do and why?

• Adds Ci::VariableType which represents the many types of variables that exist in Gitlab CI
• Adds ci_variables field to ProjectType, GroupType, and QueryType
• Includes a raw field in VariableType so we can now surface whether a variable is non-expanded

How to set up and validate locally

Use the following query in GraphQL to check each of the new variable fields:

{
  ciVariables {
    nodes {
      id
      key
      value
      variableType
      raw
      masked
      protected
    }
  }
  group(fullPath: "gitlab-org") {
    ciVariables {
      nodes {
        id
        key
        value
        variableType
        raw
        masked
        protected
      }
    }
  }
  project(fullPath: "gitlab-org/gitlab-test") {
   	ciVariables {
      nodes {
        id
        key
        value
        variableType
        raw
        masked
        protected
      }
    }
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #361934 (closed)

app/graphql/types/group_type.rb

@@ -194,6 +194,13 @@ class GroupType < NamespaceType
           complexity: 5,
           resolver: Resolvers::GroupsResolver
 
+    field :ci_variables,
+          Types::Ci::VariableType.connection_type,
+          null: true,
+          description: "List of the group's CI/CD variables.",
+          authorize: :admin_group,
+          method: :variables
+
     field :runners, Types::Ci::RunnerType.connection_type,
           null: true,
           resolver: Resolvers::Ci::GroupRunnersResolver,