Implement all types and state of subsystem

## Issue summary

Before moving forward we need to implement State and common types for the subsystem

## Implementation details

Implement:
- State of the subsystem that holds current session and PVFs (eg State, InterestView, PvfData)
- Implement methods that update state. (eg on_leaves_update, on_judgement, judgements)
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## Other information and links

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## Acceptance criteria
[] Implement state and structures
[] Implement state update and access function
[] Regression testing (if applicable)
[] New code is 60% covered with unit tests