No configuration schema

Looks good.

This still needs work.

Running tests while using this module triggers this error:

Drupal\Core\Config\Schema\SchemaIncompleteException: Schema errors for registration_role.setting with the following errors: registration_role.setting:role_to_select.administrator variable type is integer but applied schema class is Drupal\Core\Config\Schema\String

This is because the configuration looks like this:

 Drupal\Core\Config\Config {#12183
     role_to_select: [
       "contributor" => "contributor",
       "administrator" => 0,
     ],

So, when the schema is applied contributor (the selected role) is indeed a string but administrator is 0 which is integer and breaks the expected schema.

I'm going to create a new issue with the fix.

It needs follow-up to fix form and provide post update hook to remove keys with 0 values, as discussed in slack

@gnuget nice catch!

I asked in the Drupal Slack and it seems that given that we cannot know the type of the values it is better to fix the configuration to make it match with the schema.

So in my fix instead to fix the schema I will fix how is store the configuration :-)