add pagination util and spec

lib/gitlab/pagination_util.rb

+module Gitlab
+  module PaginationUtil
+    delegate :total_count,
+             :total_pages,
+             :current_page,
+             :limit_value,
+             :first_page?,
+             :prev_page,
+             :last_page?,
+             :next_page, to: :pagination_delegate
+
+    # requires a Gitlab::PaginationDelegate instance with the default configuration.
+    # Example:
+    #    @pagination_delegate ||= Gitlab::PaginationDelegate.new(page: 1,
+    #                                                            per_page: 10,
+    #                                                            count: 20)
+    def pagination_delegate
+      raise NotImplementedError.new("Expected #{self.class.name} to implement #{__method__}")
+    end
+  end
+end

spec/lib/gitlab/pagination_util_spec.rb

+require 'spec_helper'
+
+describe Gitlab::PaginationUtil, lib: true do
+  context 'class with no pagination delegate defined' do
+    let(:pagination_class) { Class.new { extend Gitlab::PaginationUtil } }
+
+    it 'throws an error calling the method' do
+      expect { pagination_class.pagination_delegate }.to raise_error(NotImplementedError)
+    end
+  end
+
+  context 'class with no pagination delegate defined' do
+    let(:pagination_class) { Class.new { extend Gitlab::PaginationUtil } }
+    let(:pagination_delegate) {
+      Gitlab::PaginationDelegate.new(page: 1,
+                                     per_page: 10,
+                                     count: 20)
+    }
+
+    let(:delegated_methods) { %i[total_count total_pages current_page limit_value first_page? prev_page last_page? next_page] }
+
+    before do
+      allow(pagination_class).to receive(:pagination_delegate).and_return(pagination_delegate)
+    end
+
+    it 'does not throw an error' do
+      expect { pagination_class.pagination_delegate }.not_to raise_error
+    end
+
+    it 'includes the delegated methods' do
+      expect(pagination_class.public_methods).to include(*delegated_methods)
+    end
+  end
+end