refactor(ocpp20): isolate per-station state with WeakMap instead of singleton properties

refactor(ocpp20): isolate per-station state with WeakMap instead of singleton properties

Per-station state (firmware update tracking, report cache, connector status
backup) was incorrectly stored as flat properties on the singleton
OCPP20IncomingRequestService, causing cross-station pollution when multiple
stations share the same service instance.

Introduce OCPP20PerStationState interface and a WeakMap<ChargingStation, ...>
to properly isolate state per station, matching the stateless service pattern
used by OCPP16IncomingRequestService. State is lazily initialized and
automatically garbage collected when the station is released.