Wrapper around `rsip` that exposes a compact C API for integration with FreeSWITCH (or other C hosts)

[altanai]

• Provide a small, safe C ABI surface so a host (FreeSWITCH module) can receive SIP messages from Rust and instruct Rust to send SIP messages.
• Keep the Rust side responsible for networking & protocol parsing. Minimize the FFI surface and make callbacks simple.