refactor: stop using type nesting in billing extensions

[CecileRobertMichon]

Reason for Change:

Addresses breaking change in Azure SDK for Go that causes the struct nesting in vmss extensions to not work properly anymore as the Type field is now both present in VirtualMachineScaleSetExtensionProperties and VirtualMachineScaleSetExtension. See Azure/azure-rest-api-specs#7724 (comment) for more details.

Issue Fixed:

Fixes #2605

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[jackfrancis]

Strictly speaking, this doesn't fix anything, right? My interpretation is that this change uses an explicit property de-referencing expression instead of relying upon the struct-embedded child properties being accessible in the parent object. So that if/when (we know that the azure-sdk-for-go actually did remove the struct-embedded property in a future version) the struct-embedded property is no longer accessible on the parent object, our code usage is resilient.

Is that correct?

[CecileRobertMichon]

Correct. There's nothing broken at the moment in aks-engine, this is a preventative fix if you will.

[codecov]

Codecov Report

Merging #2640 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2640      +/-   ##
==========================================
- Coverage   71.79%   71.76%   -0.04%     
==========================================
  Files         131      134       +3     
  Lines       24787    25001     +214     
==========================================
+ Hits        17795    17941     +146     
- Misses       5966     6010      +44     
- Partials     1026     1050      +24

[jackfrancis]

/lgtm

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CecileRobertMichon, jackfrancis, xuto2

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