fix(ocpp): use per-subclass singleton map to prevent version collision

fix(ocpp): use per-subclass singleton map to prevent version collision

The singleton instance was stored as a single static field on the base
class. In mixed OCPP 1.6/2.0 workers, the first getInstance() call
wins — subsequent calls for a different version silently return the
wrong instance cast via 'as T'.

Replace with Map<Constructor, Instance> keyed by the concrete subclass
constructor. Each version now gets its own singleton entry. Zero
changes needed in subclasses or calling code.