feat: make cidr optional for L2 networks

[sidshas03]

• Add 'type' field to cloudstack_network resource schema
• Make 'cidr' field conditionally required based on network type
• L2 networks: cidr not required, IP parameters skipped
• L3 networks: cidr required, maintains backward compatibility
• Add CustomizeDiff validation for proper type/cidr combinations
• Update parseCIDR function to handle L2 networks without cidr
• Update resource creation logic to skip IP config for L2 networks
• Update read function to detect and set network type
• Add comprehensive test coverage for L2 networks
• Update documentation with L2 and L3 usage examples

Fixes #214