Add time_to_resolve variable mapping for native forms in GLPI 11

**Problem:**

In the legacy Formcreator plugin, it was possible to map GLPI TTR (time_to_resolve) variables as form fields. This functionality is currently missing in the native forms feature of GLPI 11.

**Use case:**

We need to display the TTR return date in forms, for example: "Return date: [time_to_resolve]", similar to how other GLPI fields like departure date can be mapped.

**Expected behavior:**

The native forms in GLPI 11 should allow mapping the time_to_resolve field as a form variable to:

- Display the TTR deadline

- Use it in calculations or conditional logic

- Reference it in form field mappings

**Current situation:**

The time_to_resolve field is not available as a mappable variable in native forms.

This would help maintain feature parity with Formcreator and support ticket management workflows requiring TTR date visibility.