Add waiting room to create programming groups (#1919)

Co-authored-by: Sebastian Serth <Sebastian.Serth@hpi.de>

db/migrate/20230920094122_create_pair_programming_waiting_users.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class CreatePairProgrammingWaitingUsers < ActiveRecord::Migration[7.0]
+  def change
+    create_table :pair_programming_waiting_users, id: :uuid do |t|
+      t.references :user, index: true, null: false, polymorphic: true
+      t.references :exercise, index: true, null: false, foreign_key: true
+      t.references :programming_group, index: true, null: true, foreign_key: true
+      t.integer :status, limit: 1, null: false, comment: 'Used as enum in Rails'
+
+      t.timestamps
+    end
+  end
+end