Have standardized template/function for ArmCustomPatchSync to define optional patch model based original one #3914

Clear and concise description of the problem

With this issue discussed, in custom patch we have to re-construct a new model because Azure.ResourceManager.Foundations.ResourceUpdateModel<Employee, EmployeeProperties> does not recursively change the typegraph to include only updateable properties and make them optional. This is not ideal because we have to duplicate every properties based existing ones and we may forget to update them when new properties are added to original one.

So we could have another either template or function to standardize the way to define patch model for better maintenance.

Checklist

• Follow our Code of Conduct
• Check that this issue is about the Azure libraries for typespec. For feature request in the typespec language or core libraries file it in the TypeSpec repo
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.