azurerm_[linux|windows]_virtual_machine - add Data Disks support

[jackofallops]

This PR creates an Opt-in beta for allowing the use of Data Disks in-line with the Linux and Windows Virtual Machine resources. Support for local disk creation and attachment of existing disks is included.

create disks are new disks created with the VM and can be partially managed (e.g. can be grown)
attach disks are exiting managed disks, which cannot be managed via the VM configuration.

By default, create data disks are deleted on VM deletion, which is controlled by the features block in the provider configuration. attach disks are always left intact by the VM.

resolves #6117
Fixes #8794

[katbyte]

LGTM 👍

[tombuildsstuff]

Taking a look through on the whole this looks pretty good - I've left some comments inline, mostly fairly minor around naming - but main issue here is around the context, where we're firing both the Update and Read contexts off in the Update, where the Read has a smaller timeout (and these are staggered) - instead we should pass through the Update context through the entire Update method, so that we're using that timeout for this entire process. Also worth noting there's no docs for this at this time?

[jackofallops]

Closing this for the time being due to technical limitations on implementation that are going to be resolved in future, but no set timeframe is currently available. Will re-open / start afresh as soon as possible.

[github-actions]

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