Easy way to add service defaults from CLI

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

_No response_

### Describe the solution you'd like

Right now if you have an Aspirified app and want to add a new c# backend to it, you have to use dotnet new to get the service defaults template. Would be awesome if we have that functionaltiy in new or init, even if its just the single extensions.cs file (which could then be rewritten and shared for other langs to)

### Additional context

_No response_