Add GENERICSYSTEMEVENT support to RealTimeEventListener and RealTimeEvent enum

[Alex-Nalin]

Problem

The Symphony Agent API delivers GENERICSYSTEMEVENT events via the datahose
(POST /agent/v5/events/read with "eventTypes": ["GENERICSYSTEMEVENT"]), but
BDK Python 2.11.2 silently drops them:

INFO - ...abstract_datafeed_loop - Received event with an unknown type: GENERICSYSTEMEVENT

Root cause

GENERICSYSTEMEVENT is absent from the RealTimeEvent enum in
abstract_datafeed_loop.py, so _run_listener_method raises a KeyError and
returns without dispatching. Additionally, on_generic_system_event is missing
from RealTimeEventListener, leaving no interface for bot developers to
implement.

The generated model V4GenericSystemEvent and the V4Payload.generic_system_event
field already exist — only the two hand-maintained dispatch files need updating.

Proposed fix

1. Add GENERICSYSTEMEVENT = ("on_generic_system_event", "generic_system_event")
   to the RealTimeEvent enum.
2. Add async def on_generic_system_event(self, initiator, event) no-op to
   RealTimeEventListener.

Total change: 2 files, ~15 lines (with comments)