Allow overriding endpoint for VirtualMachineManagedIdentityCredential via IDENTITY_ENDPOINT env var

[JakeDern]

Feature Summary

This would allow users to override the endpoint that this credential looks at to talk to IMDS/HIMDS when running on an Azure VM or Arc Enabled Server.

Feature Description

Other azure sdk implementations including the dotnet sdk will look at the IDENTITY_ENDPOINT environment variable as an override to determine where to reach IMDS/HIMDS. See here.

This is an essential thing for any app running on an Arc enabled server in particular, because the Arc Agent configures that env var on installation so that applications can authenticate as the Arc Server correctly: https://learn.microsoft.com/en-us/azure/azure-arc/vmware-vsphere/azure-arc-agent

In the rust SDK the IMDS endpoint is currently hardcoded into the VirtualMachineManagedIdentityCredential implementation which wraps ImdsManagedIdentityCredential.

The proposal here is to allow overriding the endpoint via the IDENTITY_ENDPOINT env var either in ImdsManagedIdentityCredential or in VirtualMachineManagedIdentityCredential (not sure what the long term vision is for hierarchy here).

I'm happy to add this in myself if someone can point me to the preferred place to add it into the type hierarchy.

Use Case

No response

Alternatives

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Additional Context

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