Add Custom Layout form field component to Shesha Form Builder

[AlexanderSlavchovUX]

Description
Introduce a new form builder component called Custom Layout.

This component should behave like a form field container that includes a label, but instead of capturing its own value like a normal text field, it should allow users to drag and drop other form components inside it to build a custom field layout.

The goal is to give form designers a reusable container-type field that can group and arrange multiple child components under a single labeled section.

Expected Behavior

• The component can be dragged from the component toolbox into the form builder.
• It renders with a label like other form fields.
• It provides an internal drop zone / container area.
• Users can drag and drop other supported components into this container.
• Nested components can be arranged and configured inside the Custom Layout field.
• The Custom Layout itself should be configurable through the properties panel.

Acceptance Criteria

1. A user can drag Custom Layout onto the form canvas.
2. The component displays a configurable label.
3. A user can drag and drop other components into the Custom Layout area.
4. Nested components are persisted correctly in the form schema/configuration.
5. Nested components can be selected, configured, moved, and removed as expected.
6. The Custom Layout component can be saved, re-opened, and rendered without losing child structure.
7. Empty state styling clearly shows that the component accepts dropped child controls.
8. The component does not behave like a standard input field with its own direct value unless explicitly designed later.