[PP-1] HTTP API: new /xb/api/field-form route to load form for editing specific entity base/configurable field

Overview

Sibling issue of #3463988: HTTP API: new /xb/api/entity-form/{entity_type}/{entity}/{entity_form_mode} route to load form for editing entity fields (meta + non-meta), necessary for being able to easily change the value that a DynamicPropSource returns, by allowing the editing of a base/configurable field on the host entity.

Quoting #3454094: Milestone 0.1.0: Experience Builder Demo:

8. (stretch) As a creator, I want to map component properties to fields instead of using static content. For example, display the node title in a hero component, or use a design token (i.e. CSS variable) for the background color of the hero component.

(IOW: for CMS content editing and CMS templates (structured data) in #3459259: UX design tracker.)

👆 That is referring to the UX for choosing which existing host entity field to use, which will be unblocked by #3455975: HTTP API: update /xb-component/{component_id} to list possible prop sources for current entity context.

But after that is done, the natural next step is that the Content Creator is able to click through to editing the underlying structured data. That is what this response aims to provide.

(Saving these is out of scope — because it is out of scope for #3450586: [META] Back-end Kanban issue tracker too.)

Proposed resolution

Look at https://www.drupal.org/project/quickedit for inspiration.

User interface changes

None — we'll need a front-end sibling issue to actually use this. This is about allowing that front-end work to happen independently, about pre-emptively unblocking that.