Composer: add pimple/pimple as dependency

[nhaagen]

This PR adds pimple/pimple as composer dependency.

Usage:

• Provides the dependency injection container that is used as a service locator throughout the system.
• Will most likely be used as the DIC solution for the new initialisation scheme introduced with the component revision.

Wrapped By:

• ILIAS\DI\Container

Reasoning:

• Pimple provides a light weight and simple dependency injection container, which is yet quite powerful. The interface is modelled along \ArrayAccess, hence it can be easily mocked for testing.
• Dependency injection is ILIAS' future way to express dependencies between services and components.

Maintenance:

• The library has been stable for a long time and didn't receive updates since 2021.
• The actual code is only about 150 LOC and hence could be easily maintained by us, if necessary.

Links:

• Packagist: https://packagist.org/packages/pimple/pimple
• GitHub: https://github.com/silexphp/Pimple

[kergomard]

Dependency is accepted for ILIAS 11. Licence: MIT.

[klees]

License is MIT.