Add namespace support for VRRP instances

[vanyasvl]

Summary
Add support for configuring individual VRRP instances to operate in different network namespaces within a single keepalived process.

Problem
Currently, keepalived can only operate within a single network namespace, requiring multiple processes for multi-namespace deployments. This increases resource usage and complicates management.

Proposed Solution
Add a net_namespace option to vrrp_instance configuration blocks:

vrrp_instance VI_1 {
    state MASTER
    interface eth0
    net_namespace namespace1
    virtual_router_id 51
    priority 100
    virtual_ipaddress {
        192.168.1.10
    }
}

vrrp_instance VI_2 {
    state BACKUP
    interface eth1
    net_namespace namespace2
    virtual_router_id 52
    priority 90
    virtual_ipaddress {
        192.168.2.10
    }
}

Benefits

• Single process managing multiple namespaced VRRP instances
• Reduced memory footprint and operational complexity
• Better support for containerized and multi-tenant environments
• Simplified configuration management