add "struct event *" parameter to agentx_event_update function.
the function may be called via event_add*** functions.

Signed-off-by: Francois Dumontet <[email protected]>

- Introduce a dedicated AgentX thread using `frr_pthread`.

- Add mutex locks (`ax_mtx`, `ax_io_mtx`) to manage thread
synchronization for trap transfer and I/O operations.

- Implemented ring buffers (`ibuf_ax`) for handling
"master -> AgentX" communication, improving data handling
between threads.

- Update the SNMP read operations to use mutex locks to
ensure thread-safe execution.

- Integrated a new dedicated thread to send SNMP traps,
ensuring separation of responsibilities between the main
and AgentX threads.

- Enhanced trap handling to support multi-index traps, with
excess traps being discarded if the buffer is full,
preventing overflow.

- Enhanced trap handling to support multi-index traps.
When more than "RINGBUF_NB_TRAP" traps are pending for
transmission, subsequent traps are discarded to prevent
overflow.

This update significantly improves concurrency,
synchronization, and trap management within the AgentX
module, with added protection against socket's buffer overflow
from excessive traps. The socket's buffer overflow is leading
to process deadlock.

Signed-off-by: Francois Dumontet <[email protected]>