Extension point: custom data view / form

[hpehl]

Parent: #320

Summary

Allow resources to customize the Data tab's view and edit modes. Currently, ResourceData drives everything through the pipeline and ResourceView/ResourceForm with no way for consumers to inject custom content, filter attributes, or override save behavior.

Motivation

• Custom view items — inject non-attribute UI into the read-only view (e.g., a "Test Connection" button for datasources, a live status panel, a chart of pool statistics)
• Custom form items — add form fields not derived from metadata (e.g., password confirmation, file upload for certificates, a color picker for a theme attribute)
• Attribute filtering/reordering — control which attributes appear and in what order, or group them differently than the auto-grouping
• Custom save behavior — override or augment what happens on save (e.g., composite operations that write to multiple resources, post-save actions like "test connection")

Capabilities

• Inject view/form items — add items before, after, or between the pipeline-generated items
• Filter attributes — exclude specific attributes from the Data tab (e.g., computed attributes that are better shown elsewhere)
• Custom attribute grouping — override the auto-grouping with resource-specific groups
• Custom save handler — replace or wrap the default uic().crud().save() call

Registration

A registry keyed by Environment + AddressTemplate. When ResourceData initializes for a matching template, the registry provides the customizations.

Open Questions

• Should this be one registry with multiple optional hooks, or separate registries for view items, form items, filtering, and save behavior?
• How does this interact with the existing operation(Operation) customization on ResourceData?
• Should attribute filtering be declarative (list of attribute names to include/exclude) or functional (a predicate)?