feat: Introduce ShoppingMerchantOrderTracking

[bshaffer]

b/412700353

Note: this API version is v1beta. However, since our other Shopping Merchant APIs are all V1beta, this seems acceptable.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• ShoppingMerchantOrderTracking/samples/V1beta/OrderTrackingSignalsServiceClient/create_order_tracking_signal.php:25, tag merchantapi_v1beta_generated_OrderTrackingSignalsService_CreateOrderTrackingSignal_sync

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