Add dedicated "Menu icon" field for GenericObject types (supporting Tabler Icons)

### Feature request

Currently in GenericObject (2.14.14 with GLPI 10.0.19), there is no dedicated field to define a custom icon for each object type in the left navigation menu.

- In older versions, some users used the `comment` field as a workaround to inject Font Awesome classes.

- Since GenericObject 2.14.14 this workaround is no longer reliable.

- It would be much cleaner to have a dedicated field `menu_icon` for each GenericObject type.

### Expected behavior

- In the "Type" configuration page of GenericObject, add a field **Menu icon**.

- This field should accept a Tabler icon class (e.g. `ti ti-device-tv`, `ti ti-projector`).

- If empty, fallback to a default icon (e.g. `ti ti-box`).

- This value should be stored in DB (e.g. in `glpi_plugin_genericobject_types.menu_icon`).

### Benefits

- Better integration with GLPI’s UI (Tabler icons).

- No need to hijack the `comment` field.

- Consistency with core GLPI objects which already have dedicated icons.

### Environment

- GLPI: 10.0.19

- Plugin GenericObject: 2.14.14