feat: add common transport-level handshake for RDMA and UBSHM

[Spicy-cream]

Summary

This PR implements the transport-level handshake design discussed in apache/brpc#3432.

It introduces a common handshake framework above individual high-speed transports and migrates both RDMA and UBSHM handshake orchestration to that framework. TCP remains the control-plane transport during negotiation and is retained when a high-speed transport is unavailable or negotiation falls back.

This is a cleaned and consolidated version of #35.

Main changes

• Add AdapterTransport as the top-level transport for TCP, RDMA, and UBSHM sockets.
• Add common handshake session, framing, I/O, and adapter abstractions.
• Move RDMA handshake orchestration out of RdmaEndpoint.
• Move UBSHM handshake orchestration out of UBShmEndpoint.
• Keep transport-specific resource allocation and data-plane operations in their respective transports and endpoints.
• Preserve the existing RDMA protocol ID and registered name.
• Preserve RDMA v2/v3 and UBSHM wire formats.
• Preserve TCP fallback behavior when high-speed transport negotiation is unavailable or rejected.
• Add unit tests for framing, state transitions, partial input, fallback, compatibility, and transport-specific behavior.

Validation

• Default configuration and build: passed.
• Common transport handshake unit tests: 16/16 passed.
• WITH_RDMA=ON, WITH_UBRING=ON, BUILD_UNIT_TESTS=ON: configured and built successfully.
• RDMA unit tests: 86/86 passed.
• UBSHM/UBRING unit tests: passed.
• License, Linux, macOS, Bazel, CMake, Make, Clang, GCC, and ASan checks are covered by repository CI.

Scope

This PR contains the common transport-handshake infrastructure together with the RDMA and UBSHM migrations.

URMA migration is intentionally left for a follow-up change.