feat: add --pidfile support for access subcommand

[juliosuas]

Summary

Add --pidfile support to the cloudflared access tcp (and its aliases ssh, rdp, smb) subcommand.

Closes #723

Problem

The --pidfile flag works for cloudflared tunnel but not for cloudflared access subcommands. Users running cloudflared access tcp as a daemon need pidfile support for process management with systemd, launchd, or custom init scripts.

Solution

• Register the --pidfile flag on the access tcp subcommand (also exposed via TUNNEL_PIDFILE env var, matching the tunnel command)
• Write the PID file immediately on startup
• Clean up (remove) the PID file on exit via defer
• Reuse the same go-homedir path expansion already used by the tunnel pidfile implementation

The implementation is intentionally minimal — it follows the same pattern as the existing tunnel --pidfile support.

Usage

# Start an access TCP forwarder with pidfile
cloudflared access tcp --hostname example.com --url localhost:2222 --pidfile /var/run/cloudflared-access.pid

# Works with systemd for daemon management
# PIDFile=/var/run/cloudflared-access.pid in your .service unit

# Also works via environment variable
TUNNEL_PIDFILE=/var/run/cloudflared-access.pid cloudflared access ssh --hostname example.com

Changes

• cmd/cloudflared/access/cmd.go — added pidfile flag constant and registered the flag on the tcp subcommand
• cmd/cloudflared/access/carrier.go — added pidfile write on startup with deferred cleanup on exit