Try making server aware of block variants

[gwwar]

https://core.trac.wordpress.org/ticket/52688

Experiment, required for WordPress/gutenberg#29095

This PR exposes variations in the block type definition and adds two new labels item_link and item_link_description for post types and taxonomies.

Changes here will allow us to populate link navigation variations via register_block_type_from_metadata without needing to update the post type and taxonomy REST APIs to allow filtering by show_in_nav_menus.

Testing Instructions

• Checkout and run this branch locally with an environment of your choice. (Don't forget to npm run build)
• Test the GET REST endpoint /wp/v2/block-types. Notice how there is a new variations field returned.
• Make sure local tests pass by running npm run test:php -- --filter REST_Block_Type_Controller_Test

If you like, also follow test instructions in WordPress/gutenberg#29095.

navigationlink.mp4

Default label behavior

[gziolo]

This is looking good. I think we should plan to land it in WordPress core short after trunk is open for new features.

[gziolo]

We need a ticket in Trac to proceed further. As far as I can tell, there are only two remaining changes required:

• variations shouldn't be integrated with register_block_type_from_metadata for start
• CPT label need the default values

[gwwar]

Thanks @gziolo @TimothyBJacobs for all the reviews! I think I responded to all feedback, but let me know if I missed any items.

I also created a trac ticket: https://core.trac.wordpress.org/ticket/52688#ticket

[gziolo]

I'm adding the Trac ticket to my list of items to take care of later this week. I want to wait until WordPress 5.7 is out before committing. Nice work @gwwar ✨

[gwwar]

This landed in 380d044 💖