Add Drupal contrib picker/launcher/scaffolding #59
Description
A key feature for Drupal use is contrib module checkout and scaffolding.
This is quite different from a regular core Drupal setup, as we need a standardized core, but have to provide the correct version of the contrib module/theme in it.
The classic way to do this is with https://github.com/ddev/ddev-drupal-contrib, which is built for this.
It could probably also be done with our existing scaffolding, which has core/repos for this purpose.
This might be done with either an improvement to the ddev-drupal-core template or with a new template.