Customize header script for linux

### Checklist

- [x] I added a descriptive title
- [x] I searched open requests and couldn't find a duplicate

### What is the idea?

In https://github.com/conda/constructor/pull/982 i described that I would like to experiment with a few other workflows that might sound pretty intruisive for others.

I think the ability to provide my own header template, would be really cool to allow me to quickly iterate.

### Why is this needed?

Improved customization and sharing of ideas

### What should happen?

The addition of a new top level key, similar to the one that exists on windows, an option of the name `header_template` or other seems appropriate.

### Additional Context

I would love to reinterpret what the "force" option does