Add the possibility to run not only one sync job with the `SyncEngine` but managing more jobs.

## Proposed structure
- `IJobRegistry`
  - `GetAll()`, `Get(jobId)`, maybe `Upsert()`
- `JobManager`
  - `StartAsync(jobId)`
  - `StopAsync(jobId)`
  - `List()`
- `RunningJobHandle`
  - `Task` and `CancellationTokenSource`
