This change adds the initial implementation of the new API server in Kotlin,
replacing the old API written in Python. The implementation uses Quarkus,
RESTEasy, and Hibernate to implement the new API endpoints.

The reason for replacing the old API server is unifying the build and
deployment toolchains, reducing the number of technologies necessary to
work with OpenDC. Furthermore, we envision bundling the entire OpenDC
project into a single distributions, allowing users to launch their own
deployment trivially.

This change updates the project structure to become flattened.
Previously, the simulator, frontend and API each lived into their own
directory.

With this change, all modules of the project live in the top-level
directory of the repository. This should improve discoverability of
modules of the project.