Allow using the Drupal core robots.txt file

I also think this would be a useful feature.

Our usecase is that we have the default robots.txt, but we want to alter it with the xmlsitemap integration that is already built-in here.
But if there are core/scaffolding changes to it, we would like to get those.
If we want to do that, we need custom code instead of using this module, which I'm guessing others might be doing too.

To address this issue, I decided to add a new option in the settings form to allow using the default robots.txt file from Drupal Core

The patch at #6 looks good to me but:
- Needs an update hook for setting the value while keeping BC (default should be the textbox value for existing installations)
- Needs to modify default config? (IMHO default should be the existing scaffolding robots.txt)
- Needs tests
- Needs upgrade tests?

Does it need an update hook? After this change, use_default will be null on existing sites. This behaves the same as false so existing behaviour is unchanged.

Might not be required ATM, but will make things easier when/if config schema validation is strictly applied (in config schema is defined as boolean, and not as nullable).