Allow setting the OS disk caching option.

[ScottWatsonWork]

How to categorize this issue?
/area os
/area performance
/kind enhancement
/platform azure

What would you like to be added:
When a new vm is added the OS disk is set with no caching and I believe we could benefit from having the OS disk set to either read or read/write we should be able to set it. In AKS I believe the disks are set to read by default.

Why is this needed:
We are seeing dynatrace alerts about slow disks for both reads and writes so the idea is that enabling the caching option on the OS disk would provide a boost of performance without having to provision larger disks to get more throughput.

Speaking with internal people at SAP the Azure Machine Class supports this in the section

storageProfile:
      osDisk:
        caching: None <---HERE
        createOption: FromImage
        diskSizeGB: 50
        managedDisk:
          storageAccountType: StandardSSD_LRS

from: https://github.com/gardener/machine-controller-manager-provider-azure/blob/67546f879f31fdf6547d47c0410813f4740bd892/pkg/azure/provider/helpers/driver.go#L718

but it is not exposed to the shoot.yaml.