This change adds a new module, opendc-common, that contains
functionality that is shared across OpenDC's modules.
We move the existing utils module into this new module.

This change adds a new Pacer class that can pace the incoming scheduling
requests into scheduling cycles by allowing the user to specify a
scheduling quantum.

This change adds support for the Gradle version catalog feature in our
build configuration. This allows us to have a single file,
gradle/libs.versions.toml, which contains all the dependency versions
used in this project.

This change updates the build scripts to use type-safe project accessors
when specifying build dependencies between modules.

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.