refactor: convert default_hook.sh to golang agent

[aritrbas]

---

Overview

The network hook system has been refactored from a bash script to a two-tier architecture:

1. Native Go hooks (primary) - Always run, handle network configuration
2. Optional user scripts (secondary) - Run after Go hooks, allow custom extensions

Note: Some specialized environments (like EKS and OpenShift) may still need custom scripts for specific network configurations.

---

Execution Flow

At each hook point:

1. Native Go NetworkManagerHook executes
   ├─ System detection (first time only)
   ├─ DNS configuration (if NetworkManager)
   ├─ Network service restart (if needed)
   └─ AWS network file handling (if AWS)

2. User-provided script executes (if configured)
   └─ Custom logic from CALICOVPP_HOOK_* env var

---

Hook Points

There are 5 hook points:

Hook Point	When	Native Go Actions	User Script Env Var
BEFORE_IF_READ	Early init	No-op	CALICOVPP_HOOK_BEFORE_IF_READ
BEFORE_VPP_RUN	Before VPP starts	Fix DNS, save configs	CALICOVPP_HOOK_BEFORE_VPP_RUN
VPP_RUNNING	After VPP starts	Restart network, tweak configs	CALICOVPP_HOOK_VPP_RUNNING
VPP_DONE_OK	VPP stops (success)	Undo DNS, restore configs, restart network	CALICOVPP_HOOK_VPP_DONE_OK
VPP_ERRORED	VPP stops (error)	Undo DNS, restore configs, restart network	CALICOVPP_HOOK_VPP_ERRORED

---

Optional User Hook Scripts

All default to empty (no scripts by default):

CALICOVPP_HOOK_BEFORE_IF_READ=""   # Run before interface read
CALICOVPP_HOOK_BEFORE_VPP_RUN=""   # Run before VPP starts  
CALICOVPP_HOOK_VPP_RUNNING=""      # Run after VPP starts
CALICOVPP_HOOK_VPP_DONE_OK=""      # Run when VPP stops normally
CALICOVPP_HOOK_VPP_ERRORED=""      # Run when VPP errors

To add custom logic, set the env var:

# Example: On OpenShift platform
export CALICOVPP_HOOK_VPP_RUNNING='echo \'systemctl restart NetworkManager; sleep 5; nmcli con mod ens5 ipv4.dhcp-timeout infinity\' | chroot /host'

# Example: On EKS platform
export CALICOVPP_HOOK_VPP_DONE_OK='echo \'sudo rm /etc/systemd/network/ens5.network; systemctl daemon-reload; sudo systemctl restart systemd-networkd\' | chroot /host'

---

Native Go Hooks

The NetworkManagerHook Go module automatically handles:

1. System Detection

• Detects: systemd, systemd-networkd, NetworkManager, other network services
• Detects: AWS EC2 environment (via dmidecode)
• Runs once on initialization and logs the detected configuration

2. DNS Management (NetworkManager)

• Adds dns=none to /etc/NetworkManager/NetworkManager.conf before VPP starts in BEFORE_VPP_RUN
• Removes it when VPP stops in VPP_DONE_OK/VPP_ERRORED
• Prevents NetworkManager's DNS management from interfering with VPP

3. Network Service Restart

• Automatically restarts the detected network service after VPP starts (VPP_RUNNING) and after VPP stops (VPP_DONE_OK/VPP_ERRORED)

4. AWS Network File Handling

• Handles systemd-networkd configuration for dynamic IP interfaces that become unmanaged on AWS EC2 instances
• Saves original /etc/systemd/network/*.network file before VPP starts in BEFORE_VPP_RUN
• Fixes interfaces that become unmanaged while VPP runs in VPP_RUNNING
• Removes tweaked file and restores original configuration when VPP stops in VPP_DONE_OK/VPP_ERRORED

---