mbp-1126: Network segmentation using UDN

[mlorenzofr]

Description

Implements network segmentation using User Defined Networks (UDNs) for the qtodo application

Summary

• Adds NetworkAttachmentDefinition and UserDefinedNetwork resources for isolated network segments
• Configures AdminNetworkPolicy with ingress/egress rules to control traffic between network segments
• Updates qtodo app deployment and PostgreSQL statefulset to use the new UDN
• Adds documentation on UDN usage and configuration
• Introduces feature flag system to enable/disable UDN functionality

Test plan

• Validate feature flag enables/disables UDN resources correctly
• Verify qtodo pods attach to the UDN successfully
• Confirm AdminNetworkPolicy rules allow expected traffic and block unauthorized access
• Test qtodo application
• Review documentation