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)

spec/requests/api/graphql/ci/group_variables_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'Query.group(fullPath).ciVariables' do
+  include GraphqlHelpers
+
+  let_it_be(:group) { create(:group) }
+  let_it_be(:user) { create(:user) }
+
+  let(:query) do
+    %(
+      query {
+        group(fullPath: "#{group.full_path}") {
+          ciVariables {
+            nodes {
+              id
+              key
+              value
+              variableType
+              protected
+              masked
+              raw
+            }
+          }
+        }
+      }
+    )
+  end
+
+  context 'when the user can administer the group' do
+    before do
+      group.add_owner(user)
+    end
+
+    it "returns the group's CI variables" do
+      variable = create(:ci_group_variable, group: group, key: 'TEST_VAR', value: 'test',
+                        masked: false, protected: true, raw: true)
+
+      post_graphql(query, current_user: user)
+
+      expect(graphql_data.dig('group', 'ciVariables', 'nodes')).to contain_exactly({
+        'id' => variable.to_global_id.to_s,
+        'key' => 'TEST_VAR',
+        'value' => 'test',
+        'variableType' => 'ENV_VAR',
+        'masked' => false,
+        'protected' => true,
+        'raw' => true
+      })
+    end
+
+    it 'does not reveal the value of masked variables' do
+      create(:ci_group_variable, group: group, value: 'verysecret', masked: true)
+
+      post_graphql(query, current_user: user)
+
+      expect(graphql_data.dig('group', 'ciVariables', 'nodes')).to contain_exactly(
+        a_hash_including('value' => '********')
+      )
+    end
+  end
+
+  context 'when the user cannot administer builds' do
+    it 'returns nothing' do
+      create(:ci_group_variable, group: group, value: 'verysecret', masked: true)
+
+      group.add_developer(user)
+
+      post_graphql(query, current_user: user)
+
+      expect(graphql_data.dig('group', 'ciVariables')).to be_nil
+    end
+  end
+end