Add/wc blocks/product attribute terms

[timmyc]

All Submissions:

• Have you followed the WooCommerce Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Supersedes #22793

This branch introduces a new REST API namespace for the WooCommerce Blocks functionality: wc-blocks/v1. The first endpoint implemented here is the logic that currently exists in the Woo Blocks plugin for attributes terms wc-blocks/v1/products/attributes/%d/terms

How to test the changes in this Pull Request:

1. You can use Postman to make requests to wc-blocks/v1/products/attributes/%d/terms
2. and/or run phpunit tests/unit-tests/api/wc-blocks/products-attributes-terms.php

Other information:

For woocommerce/woocommerce-blocks#426

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

Changelog entry

Adds new wc-blocks/v1 endpoint for getting attribute terms for use in Woo Blocks

[claudiosanches]

Looks good, just some small changes.

[timmyc]

Thanks for the review @claudiosanches - added in the updates.

[claudiosanches]

LGTM!