Add support for Azure user assigned managed identity

[stevehipwell]

Is your feature request related to a problem? Please describe.
We can't configure Loki running in a Kubernetes cluster with a user assigned managed identity to access an Azure storage account. We use aad-pod-identity to give services their identity in our AKS clusters, much like IRSA in EKS, and need Loki to support this.

Describe the solution you'd like
Allow the user to provide the user assigned identity id when configuring the Azure storage settings.

Describe alternatives you've considered
n/a

Additional context
The current Azure package versions should support this based on the Thanos implementation. @Ottovsky mentioned this in #4256.

[stale]

Hi! This issue has been automatically marked as stale because it has not had any
activity in the past 30 days.

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[stevehipwell]

This issue isn't stale, there is an open PR to fix it.