[Feature]: Load a prompt from an external file and use inputs

### Feature Request

I'd like to load my prompts from external text files, so they are versioned cleanly.

The `Read File` doesn't solve my problem: The variables can't be linked to inputs in the resulting text node.

Example:

I would like `{{userInstructions}}` and `{{context}}` to be available as variables / inputs.

Is it possible to do this currently in Rivet ?

If not, what workaround do you use to properly version your prompts ?

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct