Composer: Add celtic/lti as composer dependency for ILIAS 12

[sergiosant02]

This PR adds celtic/lti as composer dependency.

General Information:

• This dependency was already used in ILIAS.
• License: LGPL v3

Usage:

• Used in LTIConsumer and LTIProvider components.

Wrapped By:

• components/ILIAS/LTIConsumer
• components/ILIAS/LTIProvider

Reasoning:

• The celtic/lti library provides a robust and feature-rich implementation of the LTI (Learning Tools Interoperability) protocol. It supports LTI versions 1.1, 1.2, and 1.3, along with various extensions, services, and updated security models.
• It simplifies the implementation of LTI in tool providers by abstracting communication, transforming data into useful objects, and enabling features like consumer key management, collaboration setups, and advanced outcomes service handling.

Maintenance:

• Actively maintained with 50 releases and the last update (v5.2.0) released 24 April 2024.

• The library is developed by a dedicated team of contributors and has consistent updates to align with evolving LTI standards.

Links:

• GitHub Repository: https://github.com/celtic-project/LTI-PHP
• Documentation: https://github.com/celtic-project/LTI-PHP/wiki
• Official Website: http://celtic-project.org/

[matthiaskunkel]

Jour Fixe, 08 DEC 2025: Dependency is accepted for ILIAS 12.