[Feature] Notification target ticket

[JeremieMercier]

Checklist before requesting a review

Please delete options that are not relevant.

• [x] I have performed a self-review of my code.
• [ ] I have added tests (when available) that prove my fix is effective or that my feature works.
• [ ] I have updated the CHANGELOG with a short functional description of the fix or new feature.
• [x] This change requires a documentation update.

Description

This PR adds support for injecting dynamic custom field values into GLPI's ticket notification templates.

• Depends on #941