Forms: remove related-posts from API calls

[simison]

Proposed changes:

• Cleans up "related posts" entry (and related options etc queries) from API calls for form feedback posts.
• Adds new filter to related posts letting developers control what custom post type API calls related posts hooks into

Before, related posts hooks into these:

After the change, it tries to hook into these:

API responses before:

API responses after:

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 2 files.

File	Coverage	Δ%	Δ Uncovered
projects/packages/forms/src/contact-form/class-contact-form-plugin.php	451/1341 (33.63%)	0.02%	1 ❤️‍🩹
projects/plugins/jetpack/modules/related-posts/jetpack-related-posts.php	123/845 (14.56%)	-0.02%	1 ❤️‍🩹

Full summary · PHP report · JS report

[enejb]

Should we make sure that related posts never return an array of "non" public post types or have some sort of allow list of post types vs return everything ?

[simison]

I'm hesitant to limit it to specific list since people might've built all sorts of things around this with custom post types — but many of the known custom post types by us we could limit it, sure. Different PR for that, tho!

[simison]

Thinking if the filter should mention rest_api somewhere, e.g. jetpack_related_posts_rest_api_allowed_post_types