add createDepositOrder mutation

app/services/build_deposit_order.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+class BuildDepositOrder
+  DEPOSIT_FEE = 0.05
+
+  attr_reader :paid_amount_cents, :user_id
+
+  def initilize(paid_amount_cents:, user_id:)
+    @paid_amount_cents = paid_amount_cents
+    @user_id = user_id
+  end
+
+  def build
+    DepositOrder.new(
+      user_id: user_id,
+      paid_amount_cents: paid_amount_cents,
+      received_amount_cents: paid_amount_cents + (paid_amount_cents * DEPOSIT_FEE)
+    )
+  end
+end