arch_release_promotion/files.py:
Add `load_release_from_json_payload()` to read a JSON payload and return
it as a Release instance.
Add the `ProjectFiles` class to aid in the synchronization of promoted
release types of a project by bundling all necessary functionality.
The special factory method `ProjectFiles.sync()` allows the creation of
a `ProjectFiles` instance and direct synchronization of the configured
amount of release versions of the project.

tests/test_files.py:
Add tests for `load_release_from_json_payload()`.
Add tests for `ProjectFiles` and all of its classmethods and instance
methods.