{AKS} Fix custom header extraction issue

[FumingZhang]

Description

This PR aims to fix the extraction of custom headers and add an option to allow multiple values to be appended to the same key.

Example: after the change, given the following option

--aks-custom-headers AKSHTTPCustomFeatures=Microsoft.ContainerService/Feature1,AKSHTTPCustomFeatures=Microsoft.ContainerService/Feature2

the request header would be

- request:
    headers:
      AKSHTTPCustomFeatures: 
      - Microsoft.ContainerService/Feature1,Microsoft.ContainerService/Feature2

which used to be

- request:
    headers:
      AKSHTTPCustomFeatures: 
      - Microsoft.ContainerService/Feature2

Testing Guide

History Notes

[Component Name 1] BREAKING CHANGE: az command a: Make some customer-facing breaking change
[Component Name 2] az command b: Add some customer-facing feature

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[yonzhan]

AKS