Add optional MPTCP support for upstream TCP sockets

[baily-zhang]

Summary

• add optional PeerOptions flags to request MPTCP for upstream TCP connects
• create upstream sockets with IPPROTO_MPTCP on Linux and fall back to plain TCP when enabled
• document how to enable the option in the proxy example

Behavior

• default behavior is unchanged
• MPTCP is opt-in via tcp_mptcp
• fallback to plain TCP remains enabled by default via tcp_mptcp_fallback
• non-Linux platforms return the existing socket error path unless fallback is enabled

Testing

• cargo fmt --all --check
• cargo test -p pingora-core --lib
• cargo check -p pingora-proxy