[Feat] BLS Implement in-memory workflow-instance state store

[pregnor]

Phase 2, core implementation

Implement the store that holds the state defined by the events, used later by the instance/state endpoints (implemented separately), and by the SSE (implemented separately).

• The in-memory state store should be able to receive events defined by the schema and update it's state state with the event content.
• If necessary queuing should be added to the insertion point of data, so events can be queued and processed async.
• Whenever the state is updated, the state store should also notify the SSE subscribers, the interface must be prepared for this, but the implementation can be deferred until the SSE ticket.
• Events pushed to subscribers should be the same JSON objects that will be sent as SSE NDJSON lines (schema from [Feat] BLS Define JSON schema for workflow-instance state #446 ).
• No persistence or replay; workflow-instance lifecycle is “current workflow-instance only” per epic.