[DataAvailability] Implement Forest Manager

The ingestion engine will add results to the forest as soon as they are discovered. There will also be a queue of pipelines that are ready to execute and a pool of workers pulling and executing these jobs.

However, not all results will be ready to start processing immediately after being added to the tree. There may be some criteria that must be met before first. See the design doc for the specific criteria.

Create the forest manager module. The manager will receive signals from the ingestion engine when blocks are sealed/finalized and each time a new result or receipt is added to the forest. On each signal, the manager will check if any new pipelines are ready to be executed. if they are, it will add them to the queue.