Provide a mechanism to be notified of new versions of a particular TemplateSpec

Not sure if there is prior precedence from other versioned things in Azure like Shared Image Gallery.

E.g. if you do:

```powershell
Get-AzTemplateSpec -Name ts-01 -Version 1.0 -ResourceGroupName rg01
```

You could get a response at the beginning like:

```
There is a new version of this TemplateSpec available: 2.0

Id : /subscriptions/...
...
```

Start with the Get-AzTemplateSpec example in the issue and compare how Azure Shared Image Gallery handles notifications for newer versions. Determine the expected behavior for requesting an older TemplateSpec version and define what the response should show when a newer version exists. Done means the notification mechanism and its scope are agreed and documented.

Written by the indexing model from the issue text.