Add filter on post/etc action links in the REST API

[rmccue]

With Gutenberg increasingly using more action links to control behaviour in the block editor, having a filter to adjust these links would be super useful. This allows developers to avoid a shotgun approach of hacking Gutenberg's UI, or using the very low-level filters to remove these actions.

This is probably useful to have as a general API to also add additional actions; e.g. for implementing something similar to Gutenberg's actions.

[TimothyBlynJacobs]

This makes sense to me. We'd probably also want a filter on get_schema_links.