Default Azure subscription ID if not provided using instance metadata

[whites11]

Which component this PR applies to?

cluster-autoscaler

What type of PR is this?

/kind bug

What this PR does / why we need it:

In CA version 1.21, on Azure, it was possible to omit the Subscription ID completely.
On startup, cluster autoscaler used to detect it using Azure Instance Metadata Service (IMDS).

It used to do so using a service in legacy-cloud-providers.

The initialization code for such service is as follows:

metadataService, err := providerazure.NewInstanceMetadataService(metadataURL)

In 1.21 this worked just fine.
In 1.22 this behaviour was broken, and then fixed with this PR (still unreleased, :()

In 1.23/master the feature was removed completely.
This PR aims at adding it back.

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Restore possibility to use IMDS with managed identities to authenticate cluster-autoscaler with Azure API.

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: whites11
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[marwanad]

we probably would want to avoid referencing the legacy-cloud-provider path.

There's another PR in flight here: #4689 so if it addresses the issue, we can close this one in favor of the other one.

[whites11]

Oh I missed that one! Good, I'll close mine then.
Thanks