Built-In Metadata Should Support Release Notes

For the built-in metadata properties listed here: https://github.com/Azure/autorest.powershell/blob/master/docs/metadata.md

The ReleaseNotes property is missing, and we would like to add that functionality. For example, the old way in our yaml:

```
- from: Azs.Keyvault.Admin.nuspec
where: $
transform: $ = $.replace('', 'AzureStack Hub Admin module generated with https://github.com/Azure/autorest.powershell - see https://aka.ms/azpshmigration for breaking changes.');
```