add create sell and buy crypto order mutations

app/graphql/mutations/create_buy_crypto_order.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+module Mutations
+  class CreateBuyCryptoOrder < BaseMutation
+    field :order, Types::BuyCryptoOrderType, null: true
+
+    argument :order, Inputs::CreateBuyCryptoOrderAttributesInput, required: true
+
+    def resolve(order:)
+      currency_id = decode_id(order[:currency_id])
+
+      ActiveRecord::Base.transaction do
+        current_user.fiat_balance.withdrawal!(order[:amount_cents])
+
+        record = BuyCryptoOrder.create!(
+          paid_amount_cents: order[:amount_cents],
+          currency_id: currency_id,
+          user_id: current_user.id,
+        )
+
+        { order: record }
+      rescue ActiveRecord::RecordInvalid => e
+        { errors: Resolvers::ModelErrors.from_active_record_model(e.record) }
+      end
+    end
+  end
+end