[Feature Request] Support for User-Assigned Managed Identity (UAMI) nodepool

## Description
Currently, worker nodes joined to an AKS cluster via AKS Flex Node do not automatically inherit or utilize the cluster's User-Assigned Managed Identity (UAMI) or Kubelet Identity to authenticate against Azure Container Registry (ACR).

While traditional AKS node pools handle `AcrPull` automatically via Kubelet Identity injection and Azure's credential provider, AKS Flex Nodes require manual workarounds such as `imagePullSecrets` or manual host-level credential setup.

## Requested Feature / Enhancement
We would like to request/verify:
1. Is there existing or planned support for AKS Flex Nodes to natively use a User-Assigned Managed Identity (UAMI) for Kubelet image pulling from ACR?
2. Can the agent/bootstrap mechanism automatically wire up the `acr-credential-provider` / IMDS integration for external or hybrid nodes running AKS Flex?

## Context / Use Case
We have AKS Flex Nodes running in our infrastructure that need to fetch private images directly from an Azure Container Registry (ACR). Having native UAMI support would allow seamless `AcrPull` permissions without storing static credentials (`imagePullSecrets`) inside Kubernetes namespaces.

## Environment Details
- **Cluster Type:** AKS with AKS Flex Node
- **Target Registry:** Azure Container Registry (ACR)

Start by reviewing the AKS Flex Node agent/bootstrap mechanism and the acr-credential-provider/IMDS integration mentioned in the issue. Determine whether external or hybrid nodes can use the cluster UAMI or Kubelet Identity to pull private ACR images without imagePullSecrets or host-level credentials. Done means documenting the supported behavior or defining the implementation scope.

Written by the indexing model from the issue text.