Widgets, views support, including changes from #3565343 and #3565448

Problem/Motivation

The module currently lacks (a widget and) standardized mechanisms for Dynamic Options and Dependent Fields.

• Widget: Make it available to the masses. :)
• Static Options: Users are currently limited to hardcoding enum values in the schema or using simple Taxonomy terms without filtering capabilities.
• No Interdependency: There is no built-in way to filter one field's options based on the value of another (e.g., "Car Model" depends on "Brand"), which is a critical requirement for complex forms.

Proposed resolution

Implement a modular "Option Source" plugin system and refactor the main widget structure.

• Option Source Plugins: Introduce a plugin manager to allow options to be fetched from various backends.
  • Views Source: Fetch options from any Drupal View, enabling advanced filtering and display logic.
  • Taxonomy Source: (Enhanced) Fetch terms from a vocabulary with robust validation.
• Dependent Fields: Implement lightweight Client-side JSON API triggers to reload dependent Options.
  • Frontend: Uses fetch() to get filtered options from a custom controller when a trigger field changes.
  • Backend: Dedicated JsonOptionsController that returns a simple JSON array (avoiding full form rebuilds).
• Widget Split: Create two distinct plugins:
  • JSON Schema Form (Static): Simple text area for raw schema input.
  • JSON Schema Form (From Reference): Dynamic AJAX-driven configuration to select "Schema Entity Reference" and "Schema Source Field".

Configuration Reference

This patch introduces advanced configuration via ui:options for dynamic sources.

1. Views-Based Options (Dynamic)

You can now populate a dropdown using a View. The source object now accepts a view parameter (format: view_id:display_id).

"product_type": {
  "type": "string",
  "ui:options": {
    "widget": "tagify",
    "source": {
      "plugin": "views",
      "config": {
        "view": "my_products_view:entity_reference_1"
      }
    }
  }
}

2. Dependent Fields (Filtering)

To filter options based on another field's value, add the array arguments key to the source config. This allows to watch a specified field name or use the @ prefix for a config value and pass their values as Contextual Filters to the View.

"brand": {
  "type": "string",
  "title": "Brand"
},
"model": {
  "type": "string",
  "title": "Model Name/Number",
  "ui:options": {
    "widget": "tagify",
    "allowCreate": true,
    "placeholder": "Select or type new...",
    "source": {
        "plugin": "taxonomy",
        "config": {
            "vocabulary": "car_models",
            "view": "my_view:car_models_filtered",
            "arguments": [
                "@vocabulary",  // Resolves to "car_models" from config
                "brand"               // Resolves to the value of the "brand" field
            ]
        }
    },
  }
},

Remaining tasks

• [ ] Fix failing tests (Note: Testing environment currently unavailable).
• [ ] Rewrite documentation.
• [ ] Final functional testing of complex View arguments.

User interface changes

• Widget Selection: "Manage Form Display" now offers two distinct widgets: Static and From Reference.
• Configuration Form: The "From Reference" widget features a responsive AJAX form where selecting a Reference field dynamically loads available Source fields (Text or JSON) from the target entity.
• Frontend: Forms now support real-time updates for dependent fields using a client-side JSON API. Changing a "Trigger" field (e.g., Brand) fetches new options for the "Target" field (e.g., Model) without reloading the page.
• Almost forgot... tested and working with Inline Entity Form too :)

API changes

• New Plugin Manager: JsonFormOptionSourceManager to handle option sources (views, taxonomy).
• New Base Class: JsonSchemaWidgetBase abstracts common logic (libraries, value handling) for both Static and Reference widgets.
• Added: json_form_widget.get_options route and controller for AJAX option fetching.

Data model changes

None. The schema storage format remains the same, only the widget configuration and option fetching logic have changed.