Block Hooks: Refactor update_ignored_hooked_blocks_postmeta to use meta_input

[tomjcafferkey]

The function update_ignored_hooked_blocks_postmeta uses the core function update_post_meta() to write _wp_ignored_hooked_blocks data to the database during an operation that is preparing a post to be inserted.

Since we have access to the incoming changes that is being prepared we can remove this database operation in favour of writing the data to the post object provided under meta_input.

Doing this means two things:

1. It allows us to store postmeta for new posts that are about to be created since they don't have an ID yet (which is information update_post_meta() needs).
2. The core controller will take care of updating postmeta in a more predictable pattern.

This is a prerequisite to adding hooked blocks to template parts at positions first_child and last_child ​https://github.com/WordPress/wordpress-develop/pull/6867

It's necessary because unlike wp_navigation post, the wp_template_part posts won't have a database entry upon first save meaning that update_post_meta won't work. To circumvent this we are now setting meta information on the $post stdClass object via the meta_input field which will get saved as post meta when its created/updated in the database.

Trac ticket: https://core.trac.wordpress.org/ticket/61495

[Bernhard Reiter]

In 58578:

Block Hooks: Refactor controller filter to use meta_input.

Prior to this changeset, the function update_ignored_hooked_blocks_postmeta() used the core function update_post_meta() to write _wp_ignored_hooked_blocks data to the database during an operation that is preparing a post to be inserted.

Since we have access to the incoming changes that are being prepared we can remove this database operation in favour of writing the data to the post object provided under meta_input.

Doing this means two things:

1. It allows us to store postmeta for new posts that are about to be created since they don't have an ID yet (which is information update_post_meta() needs).
2. The core controller will take care of updating postmeta in a more predictable pattern.

Props tomjcafferkey, bernhard-reiter.
Fixes #61495.

Committed to Core trunk in https://core.trac.wordpress.org/changeset/58578.