Allow extending or replacing individual elements of built-in admin sections

[radeknapora]

With the editor view I can hide a built-in section, and with widgets I can add my own. But there's nothing in between. If I want to change one thing inside a section, like adding a field to the product edit modal or making the Media section open a different picker, I have to hide the whole section and rebuild it from scratch: the section, its modals, the forms, validation, query invalidation. Then I maintain that copy and keep it in sync with every Medusa release.

The backend already handles this well. Workflows have hooks and additional_data, so I can extend product create/update without touching the core flow. The admin has no equivalent, so the extension stops halfway: the data is there, but there's no clean way to edit it where merchants expect it.

Two examples from my project:

• A few extra product fields (stored through a module link and additional_data). They belong in the product edit modal, but they can only live in a separate widget with its own edit drawer.
• A media library shared across products, categories and custom modules. The native Media section works fine, except it can only upload new files. To let it pick from the library I'd have to rebuild the entire section.

What would help, roughly in order of scope:

• Form extensions: register extra fields in built-in forms (product create/edit, category edit, etc.), with their values sent as additional_data. This alone would cover most cases I've seen.
• Named slots inside built-in sections and modals (e.g. product.media.picker, product.edit.form.after) where a plugin can render or replace one element.
• Stable IDs for those slots plus a documented set of what's exposed (form context, query keys), so overrides don't break on minor releases.

Open questions:

• Two plugins targeting the same slot: last one wins, or a build error?
• Should replacing be limited to the root project, as suggested in Allow Overriding of Existing Admin UI Components #8288, while plugins can only extend?

Related: #8288 (same problem with the options modal), #6324 (forking the dashboard as a workaround), #5571