[Feature] Application Gateway for Containers - Private/Internal Frontend IP Support

## Summary
Application Gateway for Containers (AGC) currently only supports public IP addresses for frontends. Please add support for a private/internal frontend IP, allowing the frontend to be bound to a private IP within a VNet subnet.

## Requested Feature
Support for:
- **Private-only frontend** — no public IP, internal traffic only
- **Dual frontend** — both a public and a private IP on the same AGC instance

Classic Application Gateway v2 already supports internal mode, so this would bring AGC to parity.

## References
- [AGC Frontends documentation](https://learn.microsoft.com/en-us/azure/application-gateway/for-containers/application-gateway-for-containers-components#application-gateway-for-containers-frontends)
- [Issue #5459](https://github.com/Azure/AKS/issues/5459) — private frontend acknowledged as unsupported

**Product:** Azure Application Gateway for Containers