Integrate signer sortition state tracking into the signer binary

[kantai]

Signer sortition state tracking was implemented in:

#4801

But it wasn't integrated into the signer binary yet.

The signer runloop should integrate this by holding a SortitionsView option in the RunLoop struct. Integration should be fairly straight forward -- I think roughly:

1. Add the sortition_state: Option<SortitionsView> field to RunLoop
2. On receiving a block proposal, if sortitions_view is None, reload it via fetch_view(). Then invoke check_proposal (in addition to the existing checks).
3. On receiving a burn block event, set sortitions_view to None.