Products Onboarding: Add the banner experiment to the app

[rachelmcr]

1. Add the experiment woocommerceios_products_onboarding_first_product_banner to ABTest.
2. Define the behavior for the control and treatment groups. (Control should not see any difference; treatment should see the banner.)

Docs: https://github.com/woocommerce/woocommerce-ios/blob/trunk/docs/EXPERIMENTS.md#run-an-ab-test

[peril-woocommerce]

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