Fix TreeView support for GLPI 11 Custom Assets

[BenLasTed]

Description

GLPI 11 moves GenericObject-based assets to the core Custom Assets system. TreeView currently only discovers its static list of item types, so migrated Custom Assets are not displayed.

This PR adds GLPI 11 Custom Asset support by:

• discovering Custom Asset classes through AssetDefinitionManager;
• restricting queries to the current asset definition because all Custom Assets share glpi_assets_assets;
• using the Custom Asset getSearchURL() and getFormURL() methods for navigation.

Testing

Tested with:

• GLPI 11.0.8
• TreeView 1.20.3
• 15 Custom Asset definitions
• 1,099 assets migrated from GenericObject

Verified:

• Custom Asset types are displayed in TreeView;
• filtering by type returns the correct assets;
• location search links work;
• individual asset links open the correct Custom Asset.

Related to #108