Support templates affecting multiple destination repos

### TL;DR

Some templates will want to touch multiple repositories to create a single service. For example, we might drop the service source code in one repo, and drop terraform files in a separate repo. Maybe we want to make this a single command to keep things simple for users. Our current solution would be just to ask the user to install two separate templates in different repos.

This task includes both designing and implementing such a thing.

### Detailed design

_No response_

### Alternatives considered

_No response_

### Additional information

_No response_