Expose API to programmatically validate all required fields

[johannesschaffer]

Use case: Implementing a custom "Draft" button in the Admin UI, which puts my frontend (Next.js) in Draft mode. I don't want to handle all fields as potentially undefined, however (which they are when just saving as a draft). Being able to programmatically validate, that all required fields are filled out & an array being e.g. 5 items long, like Payload does internally when pressing the "Publish" button, would save a lot of manual work