Implement persistent volumes in Azure

[Pokom]

The initial implementation for azure focused on CPU and memory. Cloudcost Exporter also needs to emit metrics for persistent volumes within AKS clusters. A loose set of steps to achieve this are

1. Update azure.PriceStore struct to have a map for storage
2. Extend PriceStore.PopulatePriceStore method to parse out persistent volume metrics(if feasible!)
3. Create method on PriceStore to fetch the price of a persistent volume
4. Create a disk store that emulates machine store functionality and gets all disks
5. Create new metrics to represent Storage
6. Collect and export storage metrics in aks Collect method
7. Update aks docs to include new metrics

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Useful docs

• pricing