VM: External diagnostics storage does not generate dependency

[ninjarobot]

This PR closes #959

The changes in this PR are as follows:

• Virtual Machines: External diagnostics storage does not generate dependency

I have read the contributing guidelines and have completed the following:

• [x] Tested my code end-to-end against a live Azure subscription.
• [ ] Updated the documentation in the docs folder for the affected changes.
• [x] Written unit tests against the modified code that I have made.
• [x] Updated the release notes with a new entry for this PR.
• [x] Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here: Bugfix, no doc change.

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

vm {
    name "my-vm"
    username "azureuser"
    diagnostics_support_external (Farmer.Arm.Storage.storageAccounts.resourceId "externalvmdiag")
}

[ninjarobot]

@ericsoler1 this change enables the VM builder to honor the linked resource being passed as managed or unmanaged. If only an ID is passed, it will make it unmanaged. Please review.

@isaacabraham could you also please review this?