Composer: Add celtic/lti as dependency

[eqsoft]

This PR adds celtic/lti as composer dependency.

Usage:

• components/ILIAS/LTI

Reasoning:

• ILIAS uses this library for basic LTI feature

Maintenance:

• Not to many but frequently commits
• Not to many but experienced persons

Links:

• Packagist: https://packagist.org/packages/celtic/lti
• GitHub: https://github.com/celtic-project/LTI-PHP
• Documentation: https://github.com/celtic-project/LTI-PHP/blob/master/README.md

[matthiaskunkel]

Jour Fixe, 22 JAN 2024: Decision about dependency is postponed. First, it should be tested if lib-lti1p3-core could replace 'celtic' and if LTI 1.1 should be dropped with ILIAS 10.

[klees]

This should have become part of ILIAS 10 via this PR. As always, something might have gone wrong. If you are still missing that package, please ping me.

Kind regards!