feat: Add APIs for custom attribute definitions (#2689)

db/migrate/20210722095814_add_custom_attribute_definition.rb

@@ -0,0 +1,16 @@
+class AddCustomAttributeDefinition < ActiveRecord::Migration[6.0]
+  def change
+    create_table :custom_attribute_definitions do |t|
+      t.string :attribute_display_name
+      t.string :attribute_key
+      t.integer :attribute_display_type, default: 0
+      t.integer :default_value
+      t.integer :attribute_model, default: 0
+      t.references :account, index: true
+
+      t.timestamps
+    end
+
+    add_index :custom_attribute_definitions, [:attribute_key, :attribute_model], unique: true, name: 'attribute_key_model_index'
+  end
+end