fix(rpc): Shuts down surfnet on exit rpc call

[0xzrf]

Summery

Shuts down the surfnet when calling the admin exit rpc method

Changes

• Added libc crate
• Used libc crate to call SIGTERM on surfnet program (Related: fix(surfnet): terminates the cli program on sigterm #589)

Notes

I do have concern about using an unsafe function like libc::raise, but the exit rpc method has to shut down the surfnet, and that felt like the most convenient way to do it. Happy to make changes if suggested

Closes #609

[MicaiahReid]

I vibe coded #612 as an alternative to this, curious to get your thoughts. AFAIK the unsafe rust here is fine, but now an exit RPC call shuts down whatever the process is. This makes sense for surfpool as a CLI, but what about surfpool as an SDK? If someone is using surfpool as an SDK in a greater tool/program, and they send the exit signal, they probably aren't wanting their whole program to shut down, just the surfnet component.

#612 will limit to just killing the surfnet + CLI