Extension point: custom resource header

[hpehl]

Parent: #320

Summary

Allow resources to replace or augment the default ResourceHeader (title + stability label + description) with resource-specific content. Currently, the header is driven entirely by the template's last segment and the resource metadata.

Motivation

• Status indicators — show a datasource's connection test result, a deployment's enabled/disabled state, or a server's running/stopped status directly in the header
• Action buttons — add resource-specific quick actions in the header area (e.g., "Test Connection", "Enable/Disable", "Restart")
• Rich header content — show additional context beyond name + description (e.g., JNDI name for datasources, bound address for socket bindings)

Capabilities

• Replace the header entirely — provide a custom IsElement<HTMLElement> that replaces the default ResourceHeader
• Augment the header — add extra content (badges, action buttons, status indicators) while keeping the default title/stability/description

Registration

A registry keyed by Environment + AddressTemplate. When ResourceShell.addHeader() is called, the registry can intercept and provide a custom header for matching templates.

Open Questions

• Should augmentation be positional (before title, after title, after description) or free-form?
• Should the custom header receive the loaded ModelNode data for dynamic content, or load its own?