feat: make OverlayExtensionConfig `ExtensionIPRange` field immutable

[robogatikov]

Reason for Change: We want to make OverlayExtensionConfig extensionIPRange field immutable to prevent accidental changes to the field. Network Containers are getting programmed for every IP address in this field's CIDR range, and DNC/DNC-RC will not support updating this field.

Issue Fixed:

Requirements:

• [ ] uses conventional commit messages
• [ ] includes documentation
• [ ] adds unit tests
• [ ] relevant PR labels added

Notes: