rgw/beast: use strand executor for timeout timer to prevent concurrent socket access

rgw/beast: use strand executor for timeout timer to prevent concurrent socket access

The timeout_timer in AsioFrontend::on_accept() was constructed with
context.get_executor(), which dispatches the timeout handler to the
raw io_context. Since each connection's coroutine runs on a strand
(via make_strand(context)), this allowed the timeout handler to
execute concurrently with the coroutine on a different thread.

When the timeout fires, timeout_handler calls socket.cancel() and
socket.shutdown() racing with the coroutine's async_read_header(),
async_write(), or async_read_some() on the same socket and buffer.
This can corrupt the flat_static_buffer's internal pointers, leading
to SIGSEGV in the Beast HTTP parser.

Fix by constructing the timeout_timer with yield.get_executor(),
which returns the coroutine's strand executor. This ensures the
timeout handler is serialized with the coroutine, preventing
concurrent access to the socket and buffer.

Fixes: https://tracker.ceph.com/issues/75031
Signed-off-by: Oguzhan Ozmen <oozmen@bloomberg.net>