Let plugin plug there configuration into `kinto init` command

Most plugins have specific configuration.
We could let them hook in the `kinto init` command when installed in the venv to generate their default configuration in this file and help the user to change the mandatory ones.

Start by locating the `kinto init` command and how installed plugins currently expose configuration. Determine the project entry points involved in generating defaults and identifying mandatory settings, then define how the behavior should be verified with a plugin installed in the virtual environment.

Written by the indexing model from the issue text.