Demonstrate slot prediction with manual seal

[JoshOrndorff]

Manual seal is a really useful tool for testing runtimes in a light-weight way. Nimbus is unlike other consensus engines because it has a prediction step. Manual seal doesn't support calling arbitrary functions in response to EngineCommands. So we have a few options:

• Put some pre-processing between the RPC layer and the manual seal call to do the prediction
• Write our own manual seal that does the prediction and doesn't try to be generic over other consensus engines
• Wrap the proposer and have the wrapper do the prediction when manual seal calls in.

In any case, this will require factoring our the slot prediction function from the parachain consensus worker.

After this is designed, it should be demonstrated in the template node in this repo.