Add OnManagerSearch event to allow extending quick search

[biz87]

Summary

Closes #16912

Adds the OnManagerSearch system event that fires during manager quick search (uberbar) processing. Plugins can register custom search providers through declarative configuration arrays — the processor builds and executes xPDO queries from these configs.

Key changes:

• New OnManagerSearch event (service=2, group=System) fired in Search/Search processor after built-in searches
• New searchProvider() method handles provider configs: builds xPDO queries with optional joins, custom search fields, permission checks, and custom icons/labels
• ArrayObject is used for the $providers parameter so that object identity is preserved through array_merge/extract in the event dispatch chain — all plugins share the same instance
• Upgrade script for existing installations
• No JS changes needed — the frontend already checks for label and icon fields before falling back to built-in mappings

Plugin usage example:

// Plugin on OnManagerSearch event
// $query, $providers, $maxResults, $searchInContent are available via extract()

$providers[] = [
    'class'            => 'MyExtra\Model\Product',
    'type'             => 'products',
    'label'            => 'Products',
    'icon'             => 'shopping-cart',
    'permission'       => 'myextra_view',
    'nameField'        => 'name',
    'descriptionField' => 'sku',
    'contentField'     => 'description',
    'idField'          => 'id',
    'action'           => 'myextra/product/update&id=',
    'joins'            => [
        [
            'class' => 'MyExtra\Model\Category',
            'alias' => 'Category',
            'on'    => 'Product.category_id = Category.id',
            'type'  => 'left',
        ],
    ],
    'searchFields' => ['Category.name'],
];

Provider config keys:

Key	Required	Description
class	yes	xPDO model class (validated via getTableName)
type	yes	Result group identifier
nameField	yes	Primary search/display field
action	yes	URL pattern (record ID is appended)
descriptionField	no	Secondary search/display field
contentField	no	Content field (respects quick_search_in_content)
label	no	Display label for group header
icon	no	FontAwesome icon name
permission	no	Permission to check before searching
idField	no	Primary key field name (default: id)
joins	no	Array of xPDO join configs (class, alias, on, type)
searchFields	no	Additional fields to search (e.g. from joins)

Test plan

• Verify built-in search (resources, elements, users) still works as before
• Create a test plugin on OnManagerSearch that registers a provider for an existing model (e.g. modCategory) and verify results appear in the search bar
• Verify permission check prevents results when user lacks the specified permission
• Verify label and icon display correctly in the search dropdown
• Verify joins and searchFields allow searching related model fields
• Verify malformed provider configs (missing required keys) are silently skipped
• Verify invalid xPDO class in provider config doesn't break the search
• Run upgrade on an existing 3.2.0 installation and verify the OnManagerSearch event is created

[Ibochkarev]

Potential regression for prefixed fields (Alias.field).

This method now supports dotted nameField in sorting, but the result mapping still reads:

• $record->get($nameField)
• $record->get($idField)

If config uses Product.name / Product.id, get() may not resolve these keys, which can produce empty name/id and broken _action URLs.

Please normalize field names before get() (strip alias), or select aliased columns and read those aliases.