Add feature to customize the Merchant Order Reference

[remcotolsma]

Request from customer:

Are we able to create a custom tag to be used as Merchant Order Reference in Adyen (like a dynamic invoice number)?

Internal Help Scout conversation: https://secure.helpscout.net/conversation/2282196531/25812?folderId=1425710

We'd like to add a custom reference (reference to a contract) to the Merchant Reference field, which currently consists of the Form ID and the entry ID.
Or we could use the Merchant Order Reference field (containing the entry ID right now) but that isn't the preferred field from the customers' perspective.

wp-pronamic-pay-adyen/src/PaymentRequestHelper.php

Lines 30 to 36 in 701b834

	/**
	* Merchant order reference.
	*
	* @link https://docs.adyen.com/api-explorer/#/CheckoutService/v68/post/payments__reqParam_merchantOrderReference
	* @link https://docs.adyen.com/api-explorer/#/CheckoutService/v68/post/sessions__reqParam_merchantOrderReference
	*/
	$request->set_merchant_order_reference( $payment->format_string( $config->get_merchant_order_reference() ) );

[remcotolsma]

Added a filter in 783900b, could be used like this:

<?php

add_filter(
	'pronamic_pay_adyen_merchant_order_reference',
	function( $merchant_order_reference, $payment ) {
		// Check if payment source is Gravity Forms.
		if ( 'gravityformsideal' !== $payment->get_source() ) {
			return $merchant_order_reference;
		}

		// Check if Gravity Forms API is available.
		if ( ! method_exists( 'GFAPI', 'get_entry' ) ) {
			return $merchant_order_reference;
		}

		// Check if Gravity Forms entry exists.
		$entry = GFAPI::get_entry( $payment->get_source_id() );

		if ( false === $entry ) {
			return $merchant_order_reference;
		}

		// Format merchant order reference.
		$merchant_order_reference = strtr(
			$merchant_order_reference,
			[
				'{gf_form_id}'  => $entry['form_id'],
				'{gf_entry_id}' => $entry['id'],
			]
		);

		return $merchant_order_reference;
	},
	10,
	2
);

https://github.com/pronamic/wp-pronamic-pay-adyen/blob/783900b51c6a969fcb004bb0870d58492af8c23f/docs/hooks.md#pronamic_pay_adyen_merchant_order_reference