Feature Request: function for generating idempotency_key

**The issue:**
We have a site using the [WooCommerce Square](https://wordpress.org/plugins/woocommerce-square/#description) plugin for a payment gateway that utilize the [WooCommerce Plugin Framework](https://github.com/skyverge/wc-plugin-framework). There appears to be an issue where the plugin is using function [`get_order_with_unique_transaction_ref`](https://github.com/skyverge/wc-plugin-framework/blob/master/woocommerce/payment-gateway/class-sv-wc-payment-gateway.php#L2318) from [`wc-plugin-framework/woocommerce/payment-gateway/class-sv-wc-payment-gateway.php`](https://github.com/skyverge/wc-plugin-framework/blob/master/woocommerce/payment-gateway/class-sv-wc-payment-gateway.php) to set it's idempotency_key causing double charging in case of false error responses.

**Feature Request**
We suggest adding a function to the frame work to idempotency_key that can be used by payment gateways that support them such as Square.