feat: add io_uring transport layer support

[yanglimingcn]

Add a new io_uring-based transport layer (IouringTransport) as an alternative to the existing TCP and RDMA transports, following the same architectural patterns as the RDMA implementation.

Core implementation:

• src/brpc/iouring/iouring_endpoint.h/cpp: IouringEndpoint (SocketUser subclass) that submits async read/write SQEs and reaps CQEs, with optional SQPOLL polling mode.
• src/brpc/iouring/iouring_helper.h/cpp: global io_uring ring lifecycle management, per-bthread-tag poller threads, and availability checks.
• src/brpc/iouring_transport.h/cpp: IouringTransport (Transport interface) wiring Init/Release/Reset/Connect/CutFromIOBuf(List)/ WaitEpollOut/ProcessEvent/QueueMessage/Debug/ContextInitOrDie.

Build system integration:

• CMakeLists.txt: BRPC_WITH_IOURING option; find_package(liburing); conditionally compile iouring sources and link -luring.
• BUILD.bazel / bazel/config/BUILD.bazel: brpc_with_iouring config_setting; conditional srcs/defines/linkopts/deps.
• WORKSPACE: new_local_repository for @com_github_axboe_liburing.
• bazel/third_party/liburing/liburing.BUILD: cc_library target for liburing.

Framework hooks:

• src/brpc/socket_mode.h: add SOCKET_MODE_IOURING enum value.
• src/brpc/transport_factory.cpp: register IouringTransport in TransportFactory::Create().
• src/brpc/socket.h: friend declarations for IouringEndpoint / IouringTransport.
• src/brpc/input_messenger.h: friend declarations for IouringEndpoint / IouringTransport.

Bug fixes:

• src/butil/single_threaded_pool.h: rename static member BLOCK_SIZE to POOL_BLOCK_SIZE to avoid conflict with the BLOCK_SIZE macro defined by <linux/fs.h> (pulled in via liburing.h).
• src/brpc/iouring_transport.cpp: move DECLARE_bool(usercode_in_*) inside namespace brpc{} to match the DEFINE_bool site in event_dispatcher.cpp, fixing linker undefined-reference errors.

Example and documentation:

• example/iouring_performance/: server, client, proto, CMakeLists.txt mirroring the rdma_performance example; supports WITH_IOURING=1 make flag.
• example/BUILD.bazel: Bazel targets for the new example.
• docs/cn/iouring.md: Chinese-language guide covering build, flags, architecture and comparison with RDMA.

What problem does this PR solve?

Issue Number:

#1650
#3212

Problem Summary:

What is changed and the side effects?

Changed:

Side effects:

• Performance effects:

• Breaking backward compatibility:

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
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