Add meta boxes for custom taxonomies in order edit screens

[vedanshujain]

Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

This PR adds support to display taxonomy widgets on the order edit screen for HPOS, when a taxonomy is registered for the shop_order post type. This is already done by WP core for post edit screens, so we add similar functionality for HPOS as well.

Additionally, this also fixes a bug when a taxonomy with a default term is registered for order, but it does not get automatically added to the new orders.

Closes #38560

How to test the changes in this Pull Request:

Using the WooCommerce Testing Instructions Guide, include your detailed testing instructions:

First, we are going to register a custom taxonomy for orders, by adding the following code snippet:

function register_custom_taxonomy_gh_38676() {
	register_taxonomy(
		'custom_tag',
		'shop_order',
		array(
			'labels' => array(
				'name' => 'Custom Tag',
			),
			'default_term' => 'new_tag',
		),
	);
	register_taxonomy(
		'custom_category',
		'shop_order',
		array(
			'labels' => array(
				'name' => 'Custom Category',
			),
			'default_term' => 'new_category',
			'hierarchical' => true,
		),
	);
}
add_action( 'init', 'register_custom_taxonomy_gh_38676' );

1. Switch to HPOS as an authoritative data table.
2. In the sidebar, you should see the following two widgets for custom taxonomies we registered, like so:

3. Add and remove categories and tags and click on update to make sure that changes persists, similar to how it works when posts are authoritative. By default, the tag new_tag and category new_category will already be applied to new order.

[github-actions]

Test Results Summary

Commit SHA: c8114f3

Test 🧪	Passed ✅	Failed 🚨	Broken 🚧	Skipped ⏭️	Unknown ❔	Total 📊	Duration ⏱️
API Tests	259	0	0	2	0	261	0m 48s
E2E Tests	195	0	0	10	0	205	14m 59s

---

To view the full API test report, click here.
To view the full E2E test report, click here.
To view all test reports, visit the WooCommerce Test Reports Dashboard.

[github-actions]

Hi @Konamiman,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions