Register the SAC callback on the AMQP transport *before* receiver
creation so the initial rabbitmq:active FLOW frame – sent by
RabbitMQ 4.3+ as part of the ATTACH handshake – is captured correctly.

New files:
- rabbitmq_amqp_python_client/qpid/proton/_flow_properties.py
  Low-level AMQP byte parser that scans raw frames for FLOW performatives
  and extracts the rabbitmq:active boolean from the link-state properties map.
- tests/test_quorum_queue_sac.py
  Integration tests: first consumer becomes active, second starts as
  standby, standby promoted on active close, plus validation tests.
- examples/quorum_queue_sac/example_quorum_queue_sac.py
  Runnable demo with two consumers showing live ACTIVE/STANDBY transitions.

Modified files:
- qpid/proton/_handlers.py: hook IOHandler.on_selectable_readable to
  invoke the SAC callback whenever a matching FLOW frame arrives.
- entities.py: add QuorumConsumerOptions(sac_state_handler=...) with
  validation (requires RabbitMQ 4.3+, incompatible with DirectReplyTo).
- consumer.py: _register_sac_callback() / _unregister_sac_callback()
  stored on conn.transport; registration happens before _create_receiver()
  to avoid the timing race with the initial ATTACH FLOW frame.
- connection.py: pass "4.3.0" version flag to consumer_options.validate().
- __init__.py: export QuorumConsumerOptions in the public API.
- CHANGELOG.md, examples/README.md: document the new feature.

Signed-off-by: Gabriele Santomaggio <G.santomaggio@gmail.com>

Signed-off-by: Gabriele Santomaggio <G.santomaggio@gmail.com>

Signed-off-by: Gabriele Santomaggio <G.santomaggio@gmail.com>