feat: add WebSocket-based support tunnel (coexists with don)

[edospadoni]

Summary

Add a new WebSocket-based remote support system (tunnel-client) alongside the existing OpenVPN-based "don". The two systems coexist — when the tunnel-client is mature, don will be removed.

What's new

• tunnel-client binary shipped in the core image (core/support-tunnel/)
• Node actions: start-tunnel-client, stop-tunnel-client, get-tunnel-client-status — run on individual nodes
• Cluster actions: start-tunnel-all-nodes, stop-tunnel-all-nodes, get-tunnel-all-nodes — orchestrate all nodes in parallel via agent.tasks.runp_brief()
• support-tunnel.service systemd unit with RuntimeMaxSec=7d safety net (session expiry managed server-side by MY platform)
• install-support-tunnel helper that extracts binary and plugins from the core image on each node
• users.d/nethvoice plugin for ephemeral FreePBX user provisioning
• diagnostics.d/health plugin that checks core services, Redis, modules, failed units, and TLS certs

How it works

• Credentials come from cluster/support_tunnel Redis hash
• The tunnel-client connects via WebSocket with yamux multiplexing
• Service discovery is automatic via Traefik routes (all nodes)
• User provisioning: leader creates cluster-admin + domain users, workers fetch credentials via USERS_FETCH stream
• Diagnostics collected on connection and sent to the support service

Configuration

# Set tunnel credentials (on leader node)
redis-cli HSET cluster/support_tunnel url "wss://support.my.nethesis.it/api/tunnel"
redis-cli HSET cluster/support_tunnel key "NETH-..."
redis-cli HSET cluster/support_tunnel secret "..."
redis-cli HSET cluster/support_tunnel exclude_patterns "pattern1,pattern2"
redis-cli HSET cluster/support_tunnel tls_insecure "true"

# View current config
redis-cli HGETALL cluster/support_tunnel

Usage

# All nodes
api-cli run cluster/start-tunnel-all-nodes
api-cli run cluster/get-tunnel-all-nodes
api-cli run cluster/stop-tunnel-all-nodes

# Single node
api-cli run node/1/start-tunnel-client
api-cli run node/1/get-tunnel-client-status
api-cli run node/1/stop-tunnel-client

What's unchanged (don)

All existing OpenVPN support files remain untouched: support.service, start-support-session, stop-support-session, get-support-session, etc. The new system uses separate paths (/var/lib/nethserver/support-tunnel/) and service names.