fix(serve): graceful shutdown on SIGTERM/SIGINT (exit 0)

[sebglon]

Problem

The serve command has no signal handler. When Kubernetes sends SIGTERM
to the container (e.g. during pod shutdown, rolling update, or job completion
with native sidecars), Go's default signal handling terminates the process
with exit code 2.

This causes Kubernetes to report the sidecar container as reason: Error
even when the pod/job completed successfully. See issue #176.

Fix

Add a signal handler in Serve() that:

1. Listens for SIGTERM and SIGINT on a buffered channel
2. Calls server.Shutdown() with a 5-second context to allow in-flight requests to complete
3. Returns naturally from Serve() via http.ErrServerClosed → exit code 0

Testing

# Start server in background, send SIGTERM, verify exit 0
./aws_signing_helper serve --certificate cert.pem --private-key key.pem ... --port 19911 &
PID=$!
sleep 1 && kill -TERM $PID && wait $PID
echo "Exit code: $?"  # → 0

Fixes #176