arch_release_promotion/config.py:
Add a `SyncConfig` pydantic model to track a global or per project
`backlog` (amount of versions synced), `directory` (with which to
synchronize), `last_updated_file` (to which to write a timestamp upon
changes in the synchronization directory) and `temp_in_sync_dir` (a bool
to set whether temporary data is downloaded to the synchronization
directory).
Add an optional instance of SyncConfig to both `ProjectConfig` and
`Projects`.
Add a root_validator to `Projects` to allow overriding the `SyncConfig`
of each project with implicit or global defaults if no `SyncConfig` is
provided.
Add validator for `Projects.projects` to ensure, that all
`ReleaseConfig` names of each `ProjectConfig` instances are unique.

tests/test_config.py:
Extend tests for `Projects` in accordance with changes to models.